In a group of 74 patients with multiple myeloma examined in different developmental stages of the disease the authors evaluated the clinical impact of assessment of the serum thymidine kinase (S-TK) activity. They recorded a significant relationship of S-TK values and the clinical activity of the disease (increase in 75% of the patients with the clinically active form of myeloma) and the general clinical condition ("performance status") evaluated by means of a five-grade scale. The authors tested the relationship of S-TK values and the degree of multiple myeloma, i.e. the clinical stage of the disease tested according to Durie-Salmon's classification (British Medical Research Council) and the authors' system modified according to Bataille. Individual stages of the disease (stage 1-3) differed as to the mean S-TK value and the median value and also as to the ratio of subjects with elevated values of this biochemical indicator. It was revealed that the cause of the relationship between S-TK and the stage of the disease is the increasing ratio of patients with the clinically active form of the disease in more advanced stages of myeloma. The S-TK activity in patients in the stable stage of the disease did not differ significantly in different stages (stage 1-3). From the practical aspect it is, however, an adverse finding that there is significant overlapping of S-Tk values in all mutually compared groups. According to preliminary experience S-TK is a simple readily accessible indicator related to proliferative characteristics of tumourous tissue which extends the hitherto available spectrum of evaluation criteria in multiple myeloma.