ABSTRACT We have previously shown that the total actin content of tumorigenic HeLa/fibroblast somatic cell hybrids is significantly lower than that of the non-tumorigenic hybrid cells. A measure of actin synthesis, relative to total protein synthesis, was obtained for these cells to determine whether the reduced actin content of the tumorigenic cells is due to specific suppression of actin synthesis. Actin synthesis was measured in cells labelled with [35S]methionine using DNase I affinity chroma-tography to isolate the actin quantitatively. The results show that actin synthesis is not specifically suppressed, since the relative amount of actin synthesized is constant for the tumorigenic and non-tumorigenic cell lines. The reduced actin content of the tumorigenic cells is therefore most likely to be the result of increased degradation of actin.