A modified mixed lymphocyte reaction (MLR) assay was developed to screen immunosuppressive agents. In the MLR, irradiation of splenocytes with UV light was employed for the preparation of stimulator cells, and the highest response was observed with the combination of splenocytes of C3H/He mice as a stimulator and those of C57BL/6 as a responder. The blastogenesis of the responder cells was quantified by a colorimetric method using MTT in 96-well microculture plates. For screening immunosuppressive substances, the MLR in combination with a cytotoxicity test can distinguish immunosuppressors from cytotoxic agents. The applications of this assay system to extracts of fungal culture was also described.