It is now well established that the type la of serine/threonine protein phosphatases (PP1α) may be implicated in malignant phenotype. Although the PP1α mRNA level increased in livers at preneoplastic stages of hepatocarinogenesis and all of examined rat ascites hepatomas, unexpectedly, dramatically decreased in some of primary hepatomas. In order to elucidate the low level of PP1α mRNA in some of primary hepatomas and investigate any correlation between cell growth rates and change(s) of both PP1α mRNA and PP1 activity, the present experiments were done. The most important aspects of the present study are:'' 1) The PP1α mRNA level may decreased in ascites hepatoma AH-109A cells harvested from a highly bloody ascites compared to ones harvested from a milky ascites. 2) The spontaneous PP1 activity in particulate fraction of AH-7974F showed a positive correlation with cell growth rate. 3) There was no correlation between proliferation rate of AH-7974F and the amount of PP1α catalytic subunit as well as potential PP1 activity in particulate fraction. Therefore, it is suggested that the PP1α mRNA level may decreased in the primary hepatomas with low proliferation rate which were under very bad nutritional conditions. Iran. Biomed. J. 2: 33-37, 1998.