In confirmation of prior studies, fragments of testicular interstitial (Leydig) cell tumors from BALB/c mice were found to be transplantable to H-2d(BALB/c and DBA/2) or allogeneic H-2q(DBA/1 and BUB/BnJ) recipients after organ culture, whereas noncultured fragments were accepted only in H-2drecipients. Lymphocyte cytotoxicity assays showed that while transplantability was altered by the organ culture process, tissue immunogenicity in vitro was unchanged. Cell suspensions from the same tumor were also found to be transplantable to both H-2dand H-2qrecipients and occasionally even to H-2aand H-2krecipients, irrespective of whether the cells were obtained from fragments previously organ-cultured or from dispersed fresh tissue. We suggest, therefore, that rather than modifying cell surface antigens, it is possible that organ culture explanation serves to (1) allow free dispersal of the tumor cells which leads to progressive tumor growth, and/or (2) raise the ratio of dead to living cells in the transplanted fragments, which has previously been shown to direct host immune responses toward enhancement.