The mol wt of the glycoprotein(s) carrying the PLA1 antigen was examined on platelets, megakaryocytes and endothelial cells by immunoblotting with a human polyclonal anti-PLA1 antibody (BE), as well as on four different monoclonal antibodies (MoAbs; DEK-1, DEK-2C, DEK-10, and DEK-16) raised against GPIIIa, the 100,000-mol wt platelet glycoprotein known to carry the PLA1 antigen. BE reacted with PLA1 positive but not with PLA1 negative platelets. DEK-1 reacted strongly with PLA1 positive platelets but weakly with PLA1 negative platelets. The remaining three MoAbs reacted equally with PLA1 positive as well as negative platelets. BE, DEK-1, DEK-10, and DEK-16 reacted with a 120,000- as well as 100,000-mol wt band on immunoblot of PLA1 positive platelets. The 120,000-mol wt band copurified with affinity purified 100,000-mol wt GPIIIa. Megakaryocytes had a prominent 120,000- as well as 105,000-mol wt band that reacted with BE on immunoblot (the 100,000-mol wt band was not detectable). Umbilical cord endothelial cells from presumed PLA-positive infants had a prominent 100,000-mol wt band that reacted with BE, DEK-16, and DEK-1 (the 120,000-mol wt band was not visualized). The 120,000- and 100,000-mol wt PLA1-positive bands could be digested with proteolytic enzymes to 55,000- to 65,000-mol wt-resistant fragments that retain PLA1 epitopes. Further digestion with endoglycosidase-H lowered the apparent mol wt by approximately 2,000 to 6,000 daltons without affecting PLA1 reactivity. We conclude that the PLA1 antigen is present on a 120,000- as well as 100,000-mol wt glycoprotein of platelets and megakaryocytes, a 105,000-mol wt band of megakaryocytes, and a 100,000-mol wt glycoprotein of endothelial cells. We postulate that the 120,000-mol wt glycoprotein, which shares three or more epitopes with the 100,000-mol wt GPIIIa, may be a post-translational precursor of this species.