The use of viability assays to assess the effect of antineoplastic agents on cell lines and tumor cells is an important investigative tool and may have clinical relevance. Such assays require very small quantities of drugs and it is the practice of many laboratories to freeze aliquots of drugs for use in these assays as required. We have investigated the stability of 11 different agents in an ATP-based chemosensitivity assay which is being evaluated for clinical use. The results show that most drugs maintain their biological activity well when frozen at -20 degrees C for periods up to 24 months, or occasionally at room temperature. However, 4-hydroperoxycyclophosphamide and mitomycin C are exceptions to this rule, and should not be kept frozen for more than 2-3 months. Cisplatin is unstable when frozen and then thawed, but maintained activity at room temperature for at least 6 months. Since biological activity may not correlate completely with chemical stability, further studies on the effect of storage are required, but it seems unlikely that the appropriate use of frozen aliquots is a major source of error in tumor chemosensitivity assays.