We transfected Chinese hamster ovary (CHO) cells with a cloned v-mos gene (pHT25). The mos family of oncogenes has previously been shown to have serine-threonine kinase activity. This kinase activity may be required for oncogenic transformation, although its exact biological role is unknown. We found that the transfected cells had an altered morphology, a slower doubling time, and an apparent increase in the amount of a 25-kilodalton (kDa) phosphoprotein that appeared to be of low abundance. Transfection of CHO cells with a cloned temperature-sensitive mos gene (ts159) led to isolation of a cell line that showed the presence of the 25-kDa phosphoprotein at the permissive but not at the nonpermissive temperature, suggesting a direct relationship between mos activity and the presence of this phosphoprotein. The characteristics of altered morphology and depressed growth rate were reminiscent of changes seen after the activation of the cyclic AMP-dependent protein kinase (PKA) in CHO cells. However, PKA activation did not stimulate phosphorylation of this 25-kDa protein, nor was there a change in total PKA activity in these cells. We suggest that the increased presence of the 25-kDa phosphoprotein is a consequence of the v-mos transfection and that it may be involved in the change of morphology and growth rate seen in the CHO cells. Phosphorylation of this protein may be a useful marker of mos and have some functional importance in the transformation of cells by the v-mos oncogene.