Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

After a terrible January, investors had hoped to put a bad month behind them and start carving out new gains. But with the latest economic data showing a slowdown in U.S. manufacturing activity, the Dow Jones Industrials (^DJI 0.13%) didn't cooperate with that plan, starting out February with a 150-point drop as of 11 a.m. EST. Telecom stocks AT&T (T 0.53%) and Verizon (VZ 1.48%) led the way down in a broad decline among most Dow components, and Pfizer (PFE 0.29%) was the sole blue-chip component to post anything more than a minimal gain.

AT&T and Verizon both lost about 3% as AT&T offered new lower rates for high-data cell phone rate plans. Specifically, the new pricing applies to AT&T's shared 10-gigabyte monthly plans, and the company estimates savings at $40 to $100 monthly for family or small-business plans that cover four smartphones. The move could encourage some customers to upgrade, paying less than they would for four-gigabyte or six-gigabyte plans. But it also represents a departure for the wireless industry, which has historically relied on its highest-use customers to provide high-margin revenue. Investors are anxiously waiting to see whether Verizon will respond in kind, which seems likely given the direct assault that AT&T makes on Verizon's pricing scheme in its press release.

Pfizer bucked the downtrend, soaring 3.7% after announcing strong results in its PALOMA-1 study of breast-cancer drug palbociclib. The company said the drug met its primary endpoint of improving progression-free survival in advanced breast cancer among post-menopausal women. Even though the study is just a phase 2 trial, palbociclib received breakthrough-therapy status, giving it the potential for an accelerated approval process. Pfizer is starting to enroll patients for a phase 3 trial of the drug.

Given that many investors make monthly contributions toward their investment accounts, the first day of the month is often seen as having a positive bias. If the Dow doesn't recover today, it could be another warning sign that short-term traders in particular will pay attention to in charting their trading course for February.