SummarySerum and cells, collected from mice immunized by infection either with newborn larvae (NBL) or per os with infective larvae of Trichinella spiralis, were passively transferred to mice which were challenged with NBL. Serum always gave very strong protection if given before challenge but not if given within 2 h after challenge. Cells from mice immunized with NBL also gave good protection, whereas those from mice immunized per os did not. If NBL were incubated in serum from immunized mice their infectivity was reduced, but if the serum was pre‐absorbed with NBL this effect was lost. Such absorbed serum did not confer immunity on recipient mic