SUMMARY No difference in human kidney transplant survival rates could be shown between male and female recipients of transplants despite the fact that pre formed cytotoxins were found at twice the frequency among females. The survival rate of male recipients with pre formed cytotoxins was consistently lower than males without cytotoxins. With female recipients, however, in the parent to child (P:C) and sibling to sibling (S:S) situation, the presence of cytotoxins was not deleterious to transplants. Eight unusual transplants which functioned in the face of a positive cross match test were found to have been done in females. In contrast to these indications of a non deleterious effect of pre formed antibodies to female recipients, kidney transplants from cadaver donors in females with cytotoxins had as low a survival rate as in males with cytotoxins. Thus, an “enhancement” effect appeared to be present in patients with cytotoxins for 1-HL-A haplotype different transplants but this effect could not be shown with 2-haplotype different transplants.