Platelets contain proteins with biochemical properties that are well adapted to promoting hemostasis. One important adhesive protein is von Willebrand factor (vWF), which is a very large protein comprised of a series of multimers, ranging from 860,000 to over 10 million daltons. In this report we describe a second platelet protein, p-155, which has a similar unique multimeric composition. Using agarose-acrylamide gel electrophoresis, platelet p-155 was shown to be composed of multimers ranging from less than 450 Kd to many million daltons. Based on this unique structure, we propose that the native molecule be designated as multimerin. Comparison with vWF showed that multimerin contained less of the very high molecular weight multimers. Differential reduction demonstrated that the smallest multimer is a trimer, composed of three 155-Kd subunits. Platelet releasate was demonstrated to contain mainly the smaller multimers, suggesting that the larger multimers bind to the platelet surface. Other studies indicate that multimerin and vWF are the two largest platelet proteins and the only two platelet proteins exhibiting a complex, disulfide-linked multimeric composition with variability in multimer size.