SummaryBone marrow‐derived cultured mast cells (BMMC) were transferred intravenously into W/Wvmice to examine if they could reconstitute defective mucosal mast cell response or defective protective capacity against infection withStrongyloides ratti.When mast cell growth factor‐producing activity of W/Wvmice were examined, mesenteric lymph node cells obtained at 7 to 14 days after infection could produce this factorin vitroby stimulation withS. ratti‐adult worm antigen. A single injection of BMMC (1 ± 107) on day 7 post‐infection (p.i.) neither caused an increase in number of intestinal mucosal mast cells not altered the kinetics of faecal larval output (LPG). On the other hand, serial injections of BMMC (5 ± 106) from day 5 to 10 p.i. (total 3 ± 107cells) resulted in the significant increase in number of intestinal mucosal mast cells. However, this treatment too could not alter the kinetics of LPG. Therefore, adoptive transfer of BMMC could cause the increase in number of histologically detectable‐mucosal mast cells, but these cells are, by themselves, not sufficient to cause the expulsion ofS. rattiadult worms from t