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 μM 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.