Serum samples of 110 recipients of kidney transplants and the 100 donors belonging to them taken before the transplantation were examined for complement-binding antibodies against cytomegaloviruses. A clear influence of the cytomegalovirus antibody state of donor and recipient on the cytomegalovirus infection appearing after transplantation, serologically ascertained by cytomegalovirus-IgM-antibody proof or seroconversion and fourfold increase of the titre in the complement-binding reaction, respectively, could be made evident. Cytomegalovirus negative recipients of kidneys of cytomegalovirus positive donors had significantly more infections than seronegative recipients of kidneys of seronegative donors (50% vs. 16%, p less than 0.05). According to the infection rate the seropositive recipients stand with about 30% between these two groups. It seems that in these recipients the serological state of the donors has no influence. These findings make evident that seronegative recipients receive the cytomegaloviruses essentially with the transplant, whereas the cytomegalovirus infections in seropositive recipients are endogenous cytomegalovirus reactivations caused by therapy.