The results of skin grafts transplanted in immunized and nonimmunized recipients was analysed. Specific sensitization for HLA-A or B determinants shortens graft survival if the recipients were immunized by s.c. injections of leukocytes. When the recipients had been pregnant, no such influence of specific HLA-A or B sensitization could be demonstrated. The variance in mean survival times of grafts exchanged between mixed lymphocyte culture (MLC)-positive donor-recipient combinations was significantly smaller than the variance in mean survival time (MST) of grafts exchanged between MLC-negative combinations. This difference could be the result of the influence of allograft immune-activating determinants of different strength in the MLC-negative donor-recipient combinations. Also the variance in MST of grafts in immunized recipients was significantly larger than the variance in MST of grafts in nonimmunized recipients. Apart from the obvious effect of HLA-A and B sensitization, other less well documented factors must have influenced graft survival. We did not find evidence for a graft enhancing effect of B cell-specific antibodies.