The effect of pretreatment of the donor with cyclophosphamide on leukocyte content in heart. Cryostat section of heart tissue samples from untreated (▪) or cyclophosphamide-treated (□) donor rats were labeled with OX1+OX30 (CD45) and OX6 (class II MHC) MoAbs using a three-step immunoperoxidase technique. Positive cells were counted using a grid in ocular eyepiece and each bar represents the mean ± SD of 4 animals in each group.