To further characterize the mast cell-fibroblast interactions and the effects of SI mutation, we tried to analyze the adhesion of cultured mast cells to 3T3 fibroblasts in vitro. itlast cells plated onto NIH/3T3 fibroblasts showed marked adhesion within 3 0 min, which reached a plateau after 3 h. The numbers of adhered mast cells were linear over the range of lo' to 5 x 10' cells inoculated into each (2 cm2) of 24 multiwells. Adhesion required active energy production and the presence of divalent cations. It was not inhibited by an RGD-containingpeptide, an anti-LFA-I antibody, or asialofetuin. Masi cells adhered efficiently to the eight 3T3 cell lines derived from +/+ mouse embryos, but not to the eight 3T3 cell lines derived from Sl/Sld mouse embryos. Adhesion to +/+ mouse spleen-derived fibroblasts lacking mast cell-supporting activity was comparable to that to SliSld/3T3 cells. The failure of mast cells to adhere to fibroblasts with the 51 mutations was not due to a production of a diffusible inhibitor by the latter. These results indicate that production of wild type SI gene product by fibroblasts is mandatory for adhesionimigration, as well as for proliferation of mast cells on them, and that the coculture system should be useful for the biochemical and molecular analysis of these interactions.