Various micro-organisms were studied for their thymidine kinase (adenosine 5'-triphosphate:thymidine 5'-phosphotransferase, EC 18.104.22.168) (TK) activity. The sonicated cell extract of Escherichia coli K12 had a TK activity of 35-66 pmol thymidine monophosphate formed min-1 (mg protein)-1. The cell extracts of Salmonella typhimurium and Klebsiella pneumoniae showed a markedly higher (5- to 11-fold) TK activity. Somewhat lower but significant TK activity was detected in the cell extracts of Staphylococcus aureus, Streptococcus pyogenes, Bacillus subtilis and Proteus mirabilis. In contrast, weak TK activity, if any, was detected in the cell extracts of Pseudomonas aeruginosa. This was also the case with respect to the cell extracts of various actinomycetes (such as Nocardia and Streptomyces) and related organisms (such as Corynebacterium, Mycobacterium and Rhodococcus).