Abnormally low loan-loss provisions were the biggest factor in what looks like a 20% inflation of big-bank earnings in the last quarter, a veteran analyst says.

The result, says Raphael Soifer of Brown Brothers, Harriman & Co., is that bank stock prices probably have little room to rise. "The profitability of major U.S. banks generally remains above levels likely to be sustained over the long term," Mr. Soifer said.

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