The influence of grafting more than one skin transplant simultaneously on one recipient was investigated. When a mixed lymphocyte culture (MLC)-negative skin was transplanted along with an MLC-positive skin, the MLC-negative skin survived for a significantly shorter time than when transplanted alone. This indicated that the MLC-positive skin provided a stimulus that could provide help to reject the MLC-negative skin. This finding might be important clinically. When an MLC-negative transplant is given to a patient, one should not transfuse this patient with MLC-positive leukocyte-rich blood.