Rat Walker 256 carcinosarcoma, human MCF-7 cell line and a specimen of ovarian serous cystadenocarcinoma were cultured in vitro and exposed to different cytostatic drugs. The drug effects were evaluated by bioluminescence, i.e., by measuring the levels of adenosine triphosphate (ATP), the basic energy source of the living cells. The intracellular ATP was released by TCA or NRS -reagent, and the ATP levels were measured directly from an aliquot of the growth medium without any extraction or precipitation steps. ATP level was significantly correlated with cell number, viability, [3H]-thymidine incorporation and stem cell assay. The most important advantages of bioluminescence method was speed, technical simplicity and good sensitivity (about 500 cells/sample easily quantitated, the results are seen directly within a few seconds) and flexibility (any cell line and drug may be studied by many different test designs). ATP method obviously describes the "well- being" of the cultured cells. According to our experience, the ATP-bioluminescence method is a powerful alternative to any other cell growth estimation method in vitro. It can be used especially in primary screening of the cytostatic activity of any known or unknown substance as well as in attempts to select an individual drug therapy for patients with cancer.