Background : ABO incompatibility in maternal-fetal relationship has been shown to cause hemolytic disease of the newborn (HDNB); a survey which is not yet done in this locality. Aim: Frequency of ABO blood group maternal-fetal incompatibility, maternal iso-agglutinins, and immune agglutinins quantitation was carried out in Osogbo, Osun State, South-West of Nigeria. Settings and Designs : A total of 260 subjects comprising 130 postpartum mothers within the age range of 22-35 years having good obstetrics history and normal delivery, with their 130 neonate babies were used for the study. Materials and Methods : ABO cell and serum groupings were carried out on the subjects using standard antisera and cells with appropriate controls. Direct Coomb′s Test was carried out on neonate red cells. Antibody quantitation by double dilution on the maternal serum using red cells containing corresponding antigen to the antibody was determined. A titer, which is the reciprocal of the highest dilution showing agglutination by Indirect Coombs Test, was determined. Another batch of sera was pretreated with 2-mecarptoethanol before determining the titer. Statistical Analysis: The distribution study results obtained were compared in percentages, whereas the antibodies quantitation was expressed as titers using the mode of the titers for compariso-agglutininsn. Results and Conclusions : Thirty-eight percent (50) mothers were ABO incompatible with their babies, whereas 62% (80) mothers were compatible. The distribution of blood groups in the compatible population showed blood group O (45%); A (30%); B (20%); and AB (5%). Mothers O, A, and B carrying incompatible babies had a frequency of 24% each, whereas mothers AB had 28%. Serologist differences occur in maternal ABO antibodies of corresponding incompatible baby ABO antigens. A high incidence of ABO maternal-fetal incompatibility observed without detection of immune agglutinins is indicative of a rare incidence of HDNB due to ABO incompatibility in the population studied.