Shares of Pfizer's animal health business, Zoetis, surged in their trading debut after the company's IPO raised $2.2 billion — the largest IPO by a U.S. company since Facebook Inc. raised $16 billion last May.

Pfizer Inc., the world's largest drugmaker, is keeping an ownership stake of at least 80 percent in Zoetis Inc. after the initial public offering of stock. The drugmaker is very likely to cut its stake in its spinoff with more stock sales down the road, said Scott Sweet, senior managing partner at, which invests in IPOs and advises other investors on the deals.

The shares rose 18 percent, or $4.79, to $30.79 in midmorning trading Friday. The stock trades under the "ZTS" ticker symbol on the New York Stock Exchange.

The New York company sold 86.1 million shares for $26 apiece, $1 above the top end of the range it estimated. That had suggested healthy demand for Zoetis from investors.