Cell populations obtained from ovarian cancer specimens were seeded in primary culture and morphologically identified as cancer cells. Methotrexate, cytosine arabinoside, 5-fluorouracil, antinomycin D, melphalan, and adriamycin were added to the culture medium at different concentrations and for various periods of time. The results are discussed in relation to the pharmacokinetic availability of drugs in the plasma compartment of patients treated by different therapuetic regimens. Totally inactive drugs can be identified by comparing plasma levels with active concentrations while for drugs active in vitro at concentrations in the range of pharmacokinetic levels, the percentage of responders among patients might be explained by the intrinsic variability of cancer cells.