The mixed lymphocyte culture (MLC) test accurately predicts the strength of rejection of transplanted organs. Ironically, the MLC test can seldom be used in cadaver organ transplantation, where it would be most useful, because the test takes 4 to 6 days to get results. We have developed an accelerated MLC test using precultured cells, which gives interpretable results in 24-48 hr. Precultured cells become responsive to T cell growth factor, reflecting the generation of lymphoblasts during preculture. When 7 consecutive pairs of subjects were compared in the fast and conventional MLC tests, the response at 48 hr in the fast MLC was significantly correlated with the response in the conventional MLC test on days 5 and 6 (P less than 0.01). Precultured cells can be frozen in liquid nitrogen without loss of accelerated responsiveness and significant proliferation is also observed within 24 to 48 hr using frozen cultured cells. Thus, preliminary culture followed by cryopreservation provides the basis for a clinically useful fast MLC test for cadaver organ transplantation.