Rats infected 1–6 weeks with N. brasiliensis were tested at weekly intervals for delayed skin hypersensitivity and spleen cell migration inhibition. The antigen preparations used were extracts of larvae and worm tissue (somatic antigens) and metabolic antigens released into culture medium by living worms and larvae. Delayed skin reactions to and migration inhibition by the two adult antigen preparations were found from the 2nd or 3rd week of infection until the end of the experiment. No response could be demonstrated to the larval somatic antigens. With the larval metabolic antigens, a delayed skin reaction could be elicited during the 2nd and 3rd week of infection only, and the inhibition of spleen cell migration was significant from the 2nd to 4th week.