On 6 patients suffering from rubeola or morbilli we examined the portion of IgG-containing cells and the number of DNA-synthesizing lymphocytes repeatedly during the course of the disease using immunofluorescent and autoradiographic methods. In virus infections we found a marked increase of both cellpopulations by comparison with normals. During the course of the disease we observed a simultaneous increase and decrease of both IgG-containing and DNA-synthesizing lymphocytes. At the beginning of that reaction, DNA-synthesizing cells prevailed upon IgG-positive cells. In 2 cases, a small portion of IgG-containing cells was found to be in the stage of DNA-synthesis