HLA-A-B-C and DR typing was performed on sixteen individuals whose PLA1 platelet group was negative. Ten of them were immunostimulated with PLA1 antigen by pregnancy or transfusion; six of them were responders and developed an anti-PLA1 antibody. A high incidence of HLA-B8 antigen was observed in that group of responders. This observation is supported by five other cases of allo-immunization reported in previous papers.