Two siblings, a girl and a boy, and their mother were abnormally susceptible to infections with bronchopneumonias leading to chronic pulmonary atelectasis in the girl and otitis leading to necrotizing changes with deafness in the mother. The causative micro-organism most often found was Haemophilus influenzae. The serum IgG was within the normal concentration range, but showed restricted electro-phoretic heterogeneity with absence of the anodal component. Serum IgG2 and IgG4 were absent. Absence of antibodies to teichoic acid and haemagglutinating antibodies to Haemophilus influenzae polysaccharide was noted as was the case with isohaemagglutinins and heteroagglutinins against sheep cells. Lymphopenia and poor [3H]thymidine uptake by peripheral lymphocytes after PHA stimulation were demonstrable in the girl and the mother. The girl had developed an epithelial ovarian tumour. The disease was a non-X-linked recessive and expressed selective humoral and selective cellular immunodeficiency. An inherited structural or regulatory gene defect is suggested. Gamma-globulin therapy had a good clinical effect.