1. The cytotoxicities of the nephrotoxic mycotoxins, citrinin and ochratoxin A were assayed on HeLa, C3H/10T1/2, NIH/3T3, MDCK (canine kidney), and HeLa P3 cell lines, using the MTT colorimetric assay. 2. Citrinin was less toxic than ochratoxin A in all of the cell lines examined. 3. The MDCK cells were more susceptible to both citrinin and ochratoxin A, in comparison with other cell lines. 4. Dose-responses, as measured by activities of leucine aminopeptidase and alkaline phosphatase of MDCK cells, were less sensitive than MTT colorimetric assay, indicating that these enzymes were not specifically inhibited in MDCK cells. 5. The ld50 of both toxins, calculated at 72 hr of incubation, was in the same order as those reported from animal experiments using rats and mice. © 1993.