The vaccinia virus A39R protein is a member of the semaphorin family. A39R.Fc protein was used to affinity purify an A39R receptor from a human B cell line. Tandem mass spectrometry of receptor peptides yielded partial amino acid sequences that allowed the identification of corresponding cDNA clones. Sequence analysis of this receptor indicated that it is a novel member of the plexin family and identified a semaphorin-like domain within this family, thus suggesting an evolutionary relationship between receptor and ligand. A39R up-regulated ICAM-1 on, and induced cytokine production from, human monocytes. These data, then, describe a receptor for an immunologically active semaphorin and suggest that it may serve as a prototype for other plexin-semaphorin binding pairs.