The cellular and molecular immune effectors that participated in host immunity against Trichinella spiralis newborn larvae were characterized in vivo using AO rats. Donor rats were immunized with 2,000 muscle larvae orally or 11,400 newborn larvae i.v. Immune serum and cells from spleen, peripheral lymph nodes, mesenteric lymph node, thoracic duct lymph and the peritoneal cavity were obtained from donor rats 10-21 days after infection and transferred into normal recipient rats. The control recipients received either no cells and serum or normal cells and normal serum obtained from normal donors. Newborn larvae (20,000-50,000) were injected either i.v. or ip into these recipients and immunity against newborn larvae was measured either by muscle larvae burden of the recipients three weeks later or by direct recovery of newborn larvae from the peritoneal cavity of the recipients. The experiments demonstrated that immune lymphocytes conferred no protection in the recipients but that immune serum and immune peritoneal cells were protective and these effects were synergistic. Cell adherence to the cuticle and killing of newborn larvae were observed in the peritoneal cavity of immune rats. Positive fluorescence was observed on newborn larvae incubated with fractionated IgM and IgG(E) antibody isotypes. Massive deposition of antibody molecules on newborn larvae was demonstrated by scanning electron microscopy. Studies using transmission electron microscopy revealed that the larval adherent cells were stimulated macrophages, neutrophils and eosinophils.