After exposing urological tumor cells to anticancer agents in vitro, cellular esterase activity and the ability to exclude propidium iodide (PI) were examined as dual indicators of functionality or "viability." High esterase activity/PI exclusion was observed in assays in which anticancer agents failed to inhibit cellular proliferation, while low esterase activity/PI exclusion was often observed when proliferation had been significantly inhibited. In a number of instances, exposure to anticancer agents did produce significant inhibition of proliferation without lowering viability. In this setting, the recovery of proliferative capacity could be demonstrated with several transitional cell carcinoma cell lines, and this recovery was always associated with high esterase activity/PI exclusion. When the proliferation of primary urological tumor preparations was inhibited by drug exposure, estimates of elevated viability were obtained in 27 per cent of the determinations. Thus, viability estimates may be an indicator of the potential for tumor-cell recovery from exposure to anticancer agents. Moreover, the potential for recovery may explain differences between the results of chemosensitivity testing and actual clinical events by reconciling clinical failures with elevated viabilities indicative of this potential.