AbstractThe absence of seroconversion to streptolysin O in convalescent patients with mucocutaneous lymph node syndrome has been a perplexing problem for investigators who are in favor of a streptococcal etiology of the disease. In the present studies, mice subjected in the newborn period to an infection with a strain ofStreptococcus pyogenes, which fails to confer cellular immuno‐responsiveness even in adult mice, and then reinfected four to six weeks later with another strain of the same species, which has an intrinsic ability to render mature murine hosts sensitive to streptococcal antigens, expressed an enhanced cell‐mediated immunity coupled with a lack of humoral responsiveness to the same antigens. Such immunological characters are an exact counterpart of those of MCLS patients. The involvement of these immunological characters in the pathological process of the disease is discus