In a group of 74 patients with multiple myeloma the authors revealed elevated values of serum thymidine kinase (REA kit ADICO Praha, range of normal values 0-5 U/l) in 40% of the patients-incl. a group of 22 subjects examined at the time of diagnosis of the disease in 50%, and a group of 52 subjects examined in different stages of the disease in 36% of the patients. If the upper range of S-TK 10 U/l was used, the ratio of patients with a raised value declined to 15%, in selected groups to 18 and 14% resp. The authors found a satisfactory correlation of serum thymidine values and values of S-beta-microglobulin, S-albumin, with the percentage ratio of plasmocytes in bone marrow and a less significant correlation was found with the red cell sedimentation rate (in IgG and IgA type) to the index of paraprotein and the serum interleukin-6 level. The authors did not reveal significant differences of serum thymidine kinase levels with regard to age, sex and immunochemical type of M-protein and type of light chains. The authors did not reveal any correlation of thymidine kinase serum levels and haemoglobin values, S-ferritin levels, the beta 2-microglobulin index and the synthetic score of plasma cells. It was found that examination of S-thymidine kinase extends in a useful way the existing spectrum of laboratory tests which help to elucidate the individual character of multiple myeloma.