The toxicity of bilirubin was investigated in 2 neural cell lines NBR10A and N115 using a quantitative dye assay 3-(4,5 dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium biomide (MTT) as a measure of cell viability and [3H]thymidine incorporation as a measure of DNA synthesis. Short exposures (up to 2 h) to bilirubin, even up to a bilirubin-albumin molar ratio of 1.5, yielded no evidence of toxicity using these assays. At longer exposure times (24 h) a decrease in cell viability and [3H]thymidine incorporation was detected at a molar ratio of 0.8 when the bilirubin concentration was 0.1 mM or higher, whereas lower bilirubin levels at this molar ratio showed no deleterious effect. The effect of bilirubin is more pronounced at a molar ratio of 1.5 with longer incubation periods. The MTT assay showed the N115 cells appeared to be more resistant to bilirubin cytotoxicity than NBR10A cells, a finding which was not obtained from [3H]thymidine incorporation studies. This discrepancy can be explained by the fact that we are measuring two different variables; the MTT assay estimates the number of viable cells at the end of the experiment by measuring mitochondrial function whereas the [3H]thymidine assay measures the rate of DNA synthesis during the last 2 h of the experiment. The concentration effect of bilirubin is evident from the [3H]-thymidine studies in that at a molar ratio of 1.5 and bilirubin concentration of 0.075 mM or higher, there is both cell kill (decrease in DNA) and inhibition of [3H]thymidine incorporation (decrease in specific activity).(ABSTRACT TRUNCATED AT 250 WORDS)