Trading of Pfizer has been halted on the New York Stock Exchange at $29.13 as of 12:03 p.m.

Chart via Google Finance 

As to why PFE was halted, there was immediately speculation that this meant there was news on the potential Pfizer takeover of AstraZeneca. AstraZeneca has turned down two prior offers from Pfizer. AZN stock fell 1.1 percent since the PFE halt.  

However, it seems the halt is more likely due to a new drug application. Zero Hedge reports, "Pfizer to submit Palbociclib NBA w/ FDA based on final results."Palbociclib treats women with advanced breast cancer and received Breakthrough Therapy designation from the FDA last April. The designation allows companies to expedite the review process for drugs that could treat serious, life threatening conditions. Pfizer later confirmed the Palbociclib application in an official statement.

AZN stock rebounded after the news of the application became public.