The avian bursa of Fabricius provides an essential microenvironment for B cell development and Ab repertoire expansion by a gene conversion mechanism. To explore regulatory interactions between B lineage cells and the bursal microenvironment, we sought to identify genes encoding cell surface molecules selectively expressed on bursal B cells. We report in this work the identification of the chB1 gene that encodes a C-type lectin molecule that is a distant relative of the mammalian B cell differentiation Ag, CD72. The chB1 gene is expressed by intrabursal B cells and a B cell line, DT40, that also diversifies its Ig V region genes by gene conversion. Two forms of this type II membrane protein, differing in their cytoplasmic domains, are generated by the differential usage of two translational initiation sites. The longer chB1 isoform, which is the most abundant, contains a consensus immunoreceptor tyrosine-based inhibitory motif in its cytoplasmic domain. Cross-linkage of the chB1 molecules inhibits proliferation of bursal B cells and the DT40 cell line. The chB1 lectin-like molecule may thus modulate intrabursal B cell development.