The serum parvovirus-like virus (SPLV) is a ubiquitous human virus that suppresses the growth of bone-marrow stem cells in vitro. Antibody to it (anti-SPLV) was found in 28 (97%) of 29 children and young adults with haemophilia treated with clotting-factor concentrates but in only 36% of those who had received multiple blood transfusions and in 20% of age-matched controls. The increased anti-SPLV prevalence in haemophiliacs was significant and was not due to passive acquisition of antibody. Haemophiliacs in a residential school showed seroconversion and rises in anti-SPLV titre following the introduction of concentrate treatment. 10 days after receiving his first dose of factor-VIII concentrate a patient had viraemia and then an anti-SPLV IgM response. These observations show that SPLV is often transmitted in clotting-factor concentrates but not in transfused blood. Whether this transmission has any harmful effect is uncertain.