The receptor encoded by the W (c-kit) locus (W receptor) is expressed on the surface of cultured mast cells (CMC) derived from normal (+/+) mice, whereas its ligand encoded by the Sl locus (Sl ligand) is expressed on the surface of fibroblast cell lines derived from murine embryos. Involvement of W receptors and Sl ligands in attachment of CMC to fibroblasts was investigated. CMC were cocultured with fibroblasts; nonattaching CMC were removed and the remaining CMC were counted. CMC derived from mice of the W/W genotype did not express the extracellular domain of W receptors, and attachment of W/W CMC to +/+ fibroblasts was significantly impaired. Fibroblasts derived from embryos of the Sl/Sl genotype did not express Sl ligands, and the attachment of +/+ CMC to Sl/Sl fibroblasts was also impaired. The Wv and W42 alleles are point mutations at the intracellular tyrosine kinase domain. Attachment of either Wv/Wv, W/Wv, or W/W42 CMC to +/+ fibroblasts was comparable with that of +/+ CMC. Moreover, the addition of monoclonal antibody against the extracellular domain of W receptors inhibited the attachment of +/+ CMC to +/+ fibroblasts. Thus, the extracellular domain of W receptors appeared to be necessary for attachment of CMC to fibroblasts.