Every quarter, many money managers have to disclose what they've bought and sold via 13F filings. Their latest moves can shine a bright light on smart stock picks.

Today, let's look at activist investment firm Relational Investors. Based in San Diego and run by Ralph Whitworth, the company has made a name for itself by agitating for changes  at many major corporations, such as Genzyme and Home Depot (NYSE:HD). (At Home Depot, Whitworth called for the company to exit its commercial building supply business.)

The company's reportable stock portfolio totaled $5.2 billion  in value as of Sept. 30, 2012, and contained just a few dozen stocks. Indeed, the top 10 holdings make up about 81%  of the overall portfolio's value.

Interesting developments
So what does Relational Investors' latest quarterly 13F filing tell us? Here are a few interesting details:

The only new holding is an exchange-traded fund (ETF), the SPDR S&P MidCap 400 (NYSEMKT:MDY). It's an easy way to add a diversified exposure to mid-cap stocks to a portfolio, and it has outperformed the S&P 500 over the past five, 10, and 15 years.

Among holdings in which Relational Investors increased its stake was medical-testing titan Quest Diagnostics (NYSE:DGX). It's a logical move, because our planet's growing and aging population will drive demand for health-care services, which often include laboratory tests. Management expressed confidence in Quest this month by hiking  the dividend by 76%, to a yield of roughly 2%, and it doesn't hurt that the CEO recently bought  about $300,000 of the company's stock.

Relational Investors reduced its stake in companies such as Occidental Petroleum (NYSE:OXY) and Flowserve (NYSE:FLS). Occidental's revenue and earnings have been growing briskly  in recent years, but it might be challenged by a gradual decrease in demand for oil, in part due to more energy-efficient vehicles and its not being as diversified as some peers. In the meantime, though, the company has been quite successful in the oil-rich Permian Basin.

Flowserve is poised to profit as global infrastructure projects proliferate, and its recent earnings report  featured a growing backlog along with rising sales, earnings, and operating margins. Its dividend yield is low, near 1%, but it has been raising it aggressively, averaging a 24%  annual hike over the past five years, with much more room to grow.

Finally, Relational Investors' biggest closed positions included Unum (NYSE:UNM) and Zimmer Holdings (UNKNOWN:ZMH.DL). Other closed positions of interest include Western Union (NYSE:WU). Disability insurance giant Unum has been pressured by Europe's weak economy, and like several peers, has stopped selling long-term care policies as they haven't been too profitable (partly due to low interest rates). Some see it as attractively priced at recent levels.

With our economy still sputtering a bit, Western Union is busy serving the many people who don't have bank accounts. But its future is threatened by the developing electronic financial payments landscape, as more people use eBay's (NASDAQ:EBAY) PayPal, and payments by smartphone gain traction. The stock recently yielded nearly 4%, and management has upped the payout by an annual average of more than 70% over the past five years.

We should never blindly copy any investor's moves, no matter how talented the investor. But it can be useful to keep an eye on what smart folks are doing. 13-F forms can be great places to find intriguing candidates for our portfolios.

Longtime Fool contributor Selena Maranjian, whom you can follow on Twitter,owns shares of eBay. The Motley Fool owns shares of Zimmer Holdings. Motley Fool newsletter services recommend Quest Diagnostics, eBay, The Home Depot, and Western Union. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.