A 16-year-old girl with disseminated molluscum contagiosum (MC) was found to have a very low level of serum IgM, elevated levels of IgG and IgA, and a high level of IgE. She had normal numbers of peripheral blood IgM +, IgG + and IgA + B-lymphocytes but their terminal differentiation into plasma cells could not be induced by pokeweed mitogen (PWM) in vitro. On the other hand, the patient's T-cells showed normal helper functions in the PWM system and normal interferon (IFN) production in vitro. However, the IgM + B-cells can be induced to differentiate into IgM secreting cells by Epstein-Barr virus (EBV), suggesting that the genetic mechanism for synthesis of the component irnmunoglobulin proteins is present. T-cell functions were impaired, as shown by delayed type cutaneous hypersensitivity (DTH) and mitogen response. The data suggest that the selective IgM deficiency of the patient is due mainly to defects in B-cells at the terminal differentiation stage, but immunological abnormalities are present in both B and T-cell systems. Neutrophil functions examined were normal. MC was treated by intravenous injection of IFN without any side effects; however, no clinical improvement was achieved.