The kinetics of anti-T. spiralis newborn larvae (NBL) immunity and its dose effects were studied in vivo. Rats were either immunized with newborn larvae i.v. or muscle larvae per os and challenged with newborn larvae either i.v. or i.p. on day 7 up to day 27 after immunization. Immunity was assessed by examining the muscle larvae burden or the larval recovery from the peritoneal cavity. Recovered newborn larvae were further examined for cell adherence and viability. Results indicate that as early as 9 days after infection and only 3 days after newborn larvae production in vivo, specific anti-newborn larvae immunity was developed. Peritoneal cells as well as blood cells adhered to the cuticles of the larvae and killed them. When different doses of immunization were examined, it was found that 2,000 muscle larvae per os induced the strongest immunity as compared to 500, 5,000 or 6,000. Such immunity maintained its strength when challenge infection with newborn larvae reached 50,000 dosage and it declined significantly when the dose reached 100,000. This indicates that the immune cells and antibodies are not re-deployed.