Monomeric G-actin, total actin and filamentous F-actin were examined during the growth of experimental tumour hepatoma Morris 5123. Actin was measured by the inhibition of the standard DNase I from bovine pancreas. A remarkable increase in total actin, and F-actin content, as well as in the state of actin polymerization (measured by the F:G actin ratio) was shown in the cytosol of the tumour cells in the second week of the tumour growth, followed by a rapid decrease in the third week. Parallel observations were made in the cytosol of the liver and in the serum of the tumour bearing rats. The results were compared with the data obtained for control healthy rats. It was shown that hepatoma Morris tumour growth is accompanied by the changes in the actin content and polymerization, occurring also in the liver of the host animal. Only G-actin was found in the serum. It increased significantly in the second week of tumour growth as compared with the G-actin level in the serum of the control healthy rats.