A male boy is described, who suffered from an intractable diarrhoea and several infections and who died in a severe marasmic state at the age of 8 months. Immunological studies revealed a block in the normal differentiation of B cells to Ig-producing plasma cells. After the age of 5 months, however, this block disappeared, leading to a dramatic increase in circulating Ig, most pronounced in the IgM class. In the intestine, plasma cells could only be detected after the age of 5 months, and then with a marked preponderance of IgM cells. Our results thus indicate a reversible block in the normal maturation of B cells in our patient. An older brother may have had a similar disease, suggesting a possible genetic basis for the disorder.