cDNA clones for 4 rat protein phosphatase 1 (PP1) catalytic subunit isotypes, PP1 alpha, PP1 gamma 1, PP1 gamma 2 and PP1 delta, were isolated (K. Sasaki et al., Jpn. J. Cancer Res. 81, 1272-1280, 1990). Antibodies were raised to determine the physiological functions of 4 isotypes. Among these isotypes, the PP1 gamma 2 protein was detected specifically in rat testis (H. Shima et al., Adv. Prot. Phosphatases, vol. 7, in press). On the other hand, PP1 gamma 1, PP1 delta and PP1 alpha were detected as 36kDa, 37kDa and 38kDa proteins in several rat organs by immunoblot analysis. All three isotypes were found in both soluble and particulate fractions. PP1 gamma 1 was predominant in the brain and PP1 delta in the brain, lung and small intestine, but the levels of both were low in the liver and muscle. It became evident that PP1 alpha was not a predominant isotype in any tissue examined.