Deoxythymidine kinase (TK; EC 2.7.1.21) activity in the liver has been used as a marker of liver regeneration after partial hepatectomy. In this study we examined TK activity of various organs, plasma and peripheral blood mononuclear cells (PMNC) in 70% partially hepatectomized rats. TK activity of lymph nodes, small intestine, heart, lung, kidney and thymus did not increase significantly during the course of the study, except for spleen at 72 h. On the other hand, PMNC-TK and liver cystosolic TK activity increased in a parallel fashion at all times after partial hepatectomy; they began to increase 12 h after surgery and peaked 48 h post-surgery. Fractionation of PMNC into T cells and B cells revealed that both populations increased and peaked 48 h post-surgery. Plasma TK peaked 12-24 h after surgery, then declined at 36, 48 and 72 h after partial hepatectomy. This change paralleled plasma levels of aspartate aminotransferase (AST) and alanine aminotransferase (ALT). PMNC-TK activity correlated significantly with liver cystosolic TK activity 24 h (r = 0.743; P < 0.05) and 48 h (r = 0.708; P < 0.05) after partial hepatectomy. However, it did not correlate with plasma levels of TK, AST and ALT. The results indicate that in the early stage of liver regeneration PMNC-TK may provide a marker of liver regenerative processes.