The toxic effects of 6-hydroxydopamine on the human neuroblastoma cell line SK-N-SH-SY5Y(SY5Y) and the Chinese hamster ovary (CHO) cell line were measured with five viability assays. Four of the assays (attachment efficiency, plating efficiency, amino acid incorporation into acid-precipitable proteins, and Trypan Blue dye exclusion) showed higher drug susceptibility in SY5Y cells than CHO cells. Only growth inhibition (proliferation index) gave results indicating greater sensitivity in CHO cells. Over a time span of 48 hr, injured cell populations lost vital functions in the following order: attachment ability, amino acid incorporation, proliferative capacity, and dye exclusion. Recovery of each of the functions occurred in sublethally injured populations. Monitoring the extinction and recovery of vital functions permitted the accurate determination of a drug concentration (30 micrograms/ml) selectively toxic for SY5Y cells. A strong correlation was noted between relative values for amino acid incorporation 3 hr after drug treatment, attachment efficiency at 24 hr, and dye exclusion at 24 and 48 hr. We concluded that Trypan Blue dye exclusion and amino acid incorporation were suitable methods for comparing the effects of cytotoxins on different cell lines, provided they were performed at the appropriate time after treatment with the toxin. The combined techniques yield both population and individual cell data, are simple to do, and are applicable to nondividing cell populations.