We have examined the mRNA levels for the catalytic subunits of three phosphoseryl/phosphothreonyl protein phosphatases PP1alpha, PP2Aalpha and PP2C by Northern blot analysis in the three primary hepatomas and the three poorly differentiated ascites hepatomas. The mRNA levels were compared with those in regenerating rat livers. In regenerating rat livers, the mRNA levels of PP1alpha, PP2Aalpha and PP2C at 24 h after partial hepatectomy were elevated 5, 14 and 10 times as compared to those of the control livers. But in the three primary hepatomas induced by 3'-methyl-4-dimethyl-aminoazobenzene(3'-Me-DAB), 2-acetylaminofluorene(2-AAF), and induced by the Solt-Farber model, the mRNA levels of PP1alpha, PP2Aalpha and PP2C were almost the same as those of the control livers. On the other hand, in the three poorly differentiated ascites hepatomas, AH13, AH66F and AH130, PP1alpha mRNA levels were 10 times higher than that of the control livers. In these ascites hepatomas, PP2Aalpha mRNA levels were distinctively lower than that of the control livers. PP2C mRNA levels were the same in AH13 and lower in AH66F and AH130 compared to the controls. From these results, it was demonstrated that PP1alpha mRNA levels were selectively increased in poorly differentiated ascites hepatoma cells.