AbstractReceptors for the Fc portion of IgG (FcR) has been demonstrated on a small population of T cells using various reagents such as aggregated IgG and erythrocyte‐antibody rosette. The immunological role of T cells bearing the FcR (Tycells) is still unestablished, but the possibility to be suppressor and/or killer has been suggested. In the present study, Tycells were detected by using double rosette formation consisting of sheep red blood cell (SRBC) and chicken erythrocyte sensitized with IgG (chick‐EA). In order to clarify the effects of virus on T cells, the ratio of Tycells to total T cells was estimated in cases of infectious diseases and discussed. The ratio of Tycells to total T cells was increased in cases of rubella and measles, however, did not show significant difference in cases of bacterial and mycoplasma infections. This may suggest that the Tycells reflect the appearance of killer to the virus infected target cells, which is known as one of the important host defence mechanisms against viral infection. The haemagglutination inhibition (HI) antibody titer of rubella and the ratio of Ty cells examined on the 14th day of illness showed statistical inverse correlation (n=22, p<0.05). This may suggest that the Tycells function as suppressor in vivo, at least on the 14th day of illn