Cells of the [rat] Morris hepatoma 7777 cultured in vitro have elevated levels of cytoplasmic thymidine kinase similar to those found in vivo. The cell-doubling time is slightly less than 24 h. The addition of 5-fluorouracil to the culture medium (10 .mu.g/ml) inhibits cell division, stimulates thymidine kinase production and results in cell death which increases with time. Treated cells release significantly (P .ltoreq. 0.01) more thymidine kinase into the culture medium than do untreated cells; this effect can also be seen following a 60 min pulsed incubation of the cells with 5-fluorouracil. The total activity of thymidine kinase released is proportional to cell death and to the number of cells originally on the plate. The possibility of monitoring tumor response to therapy in vivo by measuring serum levels of thymidine kinase was suggested.