What: Shares of biotechnology firm ImmunoGen (Nasdaq: IMGN  ) fell as much as 11% earlier in the trading session after announcing a secondary offering after the bell last night.

So what: After the market closed yesterday, ImmunoGen priced 6.25 million shares at $16, an 8% discount to its previous closing price. The goal for ImmunoGen is to raise about $100 million, although the company wasn't forthcoming about the potential uses for its cash. The offering would raise ImmunoGen's total cash to around $275 million (including the $175 million it reported at the end of its most recent quarter), with no debt, and up its shares outstanding to more than 83 million from just 43 million in 2008.

Now what: Investors appear shocked with the share offering, but there are really no surprises here. ImmunoGen has no FDA-approved drugs and, until it does, it will burn through its precious cash. ImmunoGen has been rallying since Roche (OTC: RHHBY) reported positive results from its advanced breast cancer treatment T-DM1, which is using ImmunoGen's antibody-drug conjugates, or ADCs, to deliver a toxin to targeted cancer cells. With the stock near its highs, the offering could be a smart move for shareholders in the long run despite its dilutive effects.

Seattle Genetics (Nasdaq: SGEN  ) and ImmunoGen are the only two ADC players anywhere near commercialization. With today's offering, both now have ample cash, and plenty of big pharmaceutical partnerships to support them as they grow. Needless to say, today's move lower could represent a nice buying opportunity.

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