We showed earlier that the phosphorylation of a 38 kDa protein (p38) from rat liver plasma membrane is stimulated by ras or endogenous G-proteins. We have now estimated the level of expression of p38 in liver tissues from embryos at different stages of development, regenerating liver and also in tumor cell lines of hepatic origin. Our results indicate that the expression of p38 is negatively correlated with cell division. It is suggested that the phosphorylation of p38, an event which is regulated by ras proteins and G-proteins, could be involved in signal transduction processes associated with the inhibitory regulation of cell division.