AbstractBecause of various circumstantial evidence, we have been asserting the streptococcal etiology of mucocutaneous lymph node syndrome (MCLS), although there has been no recoveryof Streptococcus pyogenesfrom the patients at any site.The present study was designed to provide a satisfactory explanation for the failure to isolate these bacteria from patients with MCLS, by electron microscopic observation of group A ftera‐hemolytic streptococci which had been inoculated intramuscularly into mice.“Spherical bodies” were detected in biopsy specimens obtained from the infection sites from 10 – 14 days onward after infection; these were spherical, 0.5–1.5yuain diameter, devoid of cell walls, perhaps uncultivable on any culture medium, and undistinguishable from those discovered in the buffy coat from peripheral blood of MCLS patients and described as ruined etiological microorganisms by Ueno et al.The implications of these findings, particularly in terms of the etiology of MCLS, were