Chediak et al. have reported that the titer of procoagulant antihemophilic factor (AHF:C; factor VIII:C) was significantly lower in obligate carriers of classic hemophilia who were daughters of affected men (paternal carriers) than in those whose fathers were normal by history (maternal carriers). In contrast, among 113 obligate carriers of hemophilia, no significant difference in procoagulant AHF titers was observed between paternal and maternal carriers. The concentration of AHF-like precipitating antigens, however, was significantly higher in maternal than in paternal carriers. This difference may have reflected in part the greater severity of disease in affected males in the families of maternal carriers.