The neutral red (NR) and tetrazolium MTT in vitro cytotoxicity assays were compared for 28 test agents of widely varying potency using the BALB/c mouse 3T3 fibroblast cell line as the bioindicator. For any given cell density in the microtitre plate well, the optical density absorbance with the NR assay was about twice that obtained with the MTT assay. However, there was good agreement (r = 0.939) between the ranking of the test agents on the basis of midpoint cytotoxicity values (NR(50) and MTT(50)), although the assays were based on different physiological endpoints. Nevertheless, the two assays were found to differ in sensitivity for a few test agents, such as chloroquine sulphate and several antineoplastic drugs. Both assays were capable of demonstrating a reduction in the cytotoxicity of methotrexate by leucovorin.