AbstractPECAM‐1 (CD31) is a cell adhesion molecule that is highly expressed in the endothelium. Hematopoietic cells including platelets, monocytes, neutrophils, and some T cells also express moderate levels of PECAM‐1. PECAM‐1 undergoes alternative splicing generating a number of isoforms in the endothelium. However, the expression of PECAM‐1 isoforms in hematopoietic cells and platelets has not been determined. Here, we examined the expression pattern of PECAM‐1 isoforms in human and rodent hematopoietic cells and platelets by RT‐PCR and DNA sequencing analysis. Our results showed that multiple PECAM‐1 isoforms are expressed in a cell‐type and species‐specific pattern. We identified seven human PECAM‐1 isoforms, six murine PECAM‐1 isoforms, and four rat PECAM‐1 isoforms. The full‐length PECAM‐1 was the predominant isoform detected in human cells. The PECAM‐1 isoforms that lack exon 14 and 15 (Δ14&15) or Δ12,14&15 were the predominant isoform in rodent cells. In addition, we identified a novel PECAM‐1 isoform, Δ13&14, in human hematopoietic cells. Thus, hematopoietic cells express multiple isoforms of PECAM‐1 in a pattern similar to that observed in the endothelium of the same species. The regulated expression of these isoforms may be important during hematopoiesis and transendothelial migration. J. Cell. Biochem. 87: 424–438, 2002. © 2002 Wiley‐Liss, Inc.