A combination of humoral immunodeficiency and isolated growth hormone deficiency was observed in a girl with Mulibrey nanism. The humoral immunodeficiency consisted of subnormal concentration of serum IgG, in particular IgG2 and IgG4, and low concentration of serum IgM. Serum IgA and IgD were elevated, IgE was absent. Antibody response in vivo was very low or absent and opsonization in vitro was defective. Total B-cell number was low. In addition, the serum kappa/lambda light chain ratios within the immunoglobulin classes G, A, and M were abnormal. The defective antibody response may be linked to the abnormal kappa/lambda light chain ratios. Endocrine functions were normal except for isolated growth hormone deficiency. Therapy with human growth hormone resulted in increased growth velocity but did not improve humoral immune functions.