Specific Aims: In preliminary studies, we have raised anti-human platelet monoclonal antibodies and developed a new, quick and economical microcytotoxic assay for detecting platelet cell surface antigens, e.g. HLA, PlA1. Our specific aim is to explore expression of platelet cell surface antigens, through allosera and monoclonal antiboides. Specifically, we intend to 1) study differential expression of HLA alloantigens on platelets versus lymphocytes, 2) identify new anti-platelet-specific allosera through a screening program, 3) raise additional monoclonal anti-platelet antibodies and apply the human hybridoma technology to produce human monoclonal anti-plateletr antibodies, 4) study the relationship between platelet specific markers recognized by allosera versus monoclonal antibodies, 5) perform population and genetic family studies relevant to the markers recognized by allosera and monoclonal antibodies, 6) relate new markers to those already known. Methodology: We plan to 1) study yearly 150 individuals for HLA markers on platelet versus lymphocytes, 2) screen yearly over 1,000 sera from: a) multiparous women, b) multitransfused patients c) patients with platelet disorders, d) multispecific sera from the NIH serum bank, 3) Perform yearly 10 fusions, half for producing mouse and half for producing human, antiplatelet hybridomas. 4) Establish the reciprocal relationships between allosera and monoclonal antibodies through specific screenings, absorptions, and blocking experiments. Using a Chi square and ranking analysis, group sera with regard to their inter-relationships. 5) Ascertain new platelet-specific markers by studying populations and 10 HLA genotyped families using selected allosera and monoclonal antibodies; calculate gene frequencies and Mendelian inheritance patterns. 6) relate new platelet specific markers with those already known, e.g. PlA1, PlE1, Koa. Long-term objectives: The study of platelet cell surface antigens has relevance to platelet transfusion, blood banking, immune platelet disorders, etiology of platelet-endothelial cell and vascular smooth muscle cell interaction in thrombosis and atherosclerosis. Thus, the long-term goals are toward improved management of platelet resources, reduction in morbidity and mortality due to platelet disorders and improved understnading of the immunogenetics of tissue specific antigens.