This report describes the use of an in vivo 51Cr release assay which is able to monitor various immune reactions in the rat. This assay is able to quantitate the immune response to tumour and also to assess the relative efficacy of different immunostimulants in this tumour model. These data correlate well with survival studies in tumour-bearing rats. This assay can also measure the sensitizing effect of antigens from animals differing at both the major and minor histocompatibility locus using various routes of sensitization and various donor tissues. Allograft survival can also be correlated to in vivo 51Cr release. This method is both reproducible and specific. The in vivo 51Cr release assay is a useful and versatile means of monitoring the development of antitumour responses and the degree of sensitization to histocompatibility antigens in a variety of situations in the rat. It may therefore prove useful as a rapid method for screening new antitumour and immunosuppressive regimens to be used in various studies.