SUMMARYMice chronically infected withSchistosoma mansoniexhibited a significant resistance to a second infection with the same parasite, as demonstrated by their challenge worm burdens measured by portal perfusion. A decreased worm recovery was also exhibited by chronically infected mice when the challenge was administered intravenously using 3‐h schistosomula obtained by the isolated skin technique or using 5‐, 6‐, 7‐ and 9‐day‐old schistosomula obtained from the lungs ofinfected donor mice. Variable results were obtained with 10‐ and II‐day‐old forms, while schistosomula which were 12days old or older, did not undergo significant rejection when introduced into the mesenteric veins of preinfected mice. Attempts to analyse these phenomena using the‘lung assay’were made complicated by the observation that the day of maximum recovery from the lungs was dependent upon the