The type 2A protein phosphatases in mammalian tissue extracts are inhibited completely and specifically by 1-2 nM okadaic acid. In contrast, type 1 protein phosphatases are hardly affected at these concentrations, complete inhibition requiring 1 microM okadaic acid. These observations have been exploited to develop an improved procedure for the identification and quantitation of type 1, type 2A and type 2C protein phosphatases in tissue extracts.