ABSTRACTIn order to elucidate the immunological mechanism in the pathogenesis of pityriasis rosea, immunofluorescent studies were performed on sera obtained from forty patients with this disease. Antibodies against the cytoplasm of normal human epidermal cells were demonstrated in the sera of all patients. The antibody titer showed a tendency to increase within 3 weeks after onset of secondary eruptions and then to decrease gradually until the period of recovery. The immunoglobulin class was determined to be IgM. Furthermore, by the direct immunofluorescent technique, deposits of IgM in the epidermal cells of skin lesions were demonstrated in 3 of 6 herald lesions and in 1 of 4 secondary eruptions.It is suggested that anti‐cytoplasmic antibodies produced by some unknown cause may induce the development of secondary eruptions of this diseas