The development of sensitive immunofluorescent technics for the detection of platelet antibodies has stimulated renewed interest in the field of platelet immunology. However, the analysis of multispecific sera containing both HLA and platelet-specific antibodies requires cumbersome absorption technics and relatively large quantities of serum. In this report we describe the feasibility of screening sera from multiparous women and multitransfused patients for platelet-specific antibodies using blocking inhibition with a murine monoclonal antibody against HLA A, B, and C antigens followed by platelet immunofluorescence. Platelet immunofluorescence detected anti-A antibodies in sera from some blood group O donors. Known HLA antibodies usually were detectable by platelet immunofluorescence, but false-positive reactions occurred frequently. Analysis of sera from recipients of multiple platelet transfusions revealed that 47/78 (60.2%) were positive by platelet immunofluorescence. After blocking inhibition-persistent fluorescence was observed with 11/29 sera (37.9%), suggesting the presence of platelet-specific antibodies. Absorption studies performed on two of these sera revealed that antibody activity was removed by absorption with platelets but not lymphocytes. Platelet immunofluorescence combined with monoclonal antibody blocking inhibition proved to be a valuable tool to distinguish between HLA and platelet-specific antibodies.