The metabolism of albumin and IgG was investigated in two siblings, products of a first-cousin marriage, a female aged 34 yr and a male aged 17, who had a marked reduction in their respective serum concentrations of IgG (1.3 and 3.1 mg/ml) and albumin (19 and 21 mg/ml). The metabolism of radioiodinated IgG and albumin was studied in the two patients. The total circulating and body pools of IgG were less than 28% of normal. The IgG synthetic rates were within the normal range. However, the IgG survival was short, with their respective fractional catabolic rates increased fivefold to 31% and 36% of the intravenous pool per day (normal, 6.7 +/- 2%/d). Furthermore, the patients had reduced total body pools, normal synthetic rates, and increased fractional catabolic rates for albumin. There was no proteinuria or abnormality of renal or liver function. In addition, the patients did not have circulating antibodies directed toward IgG, IgA, or albumin. Furthermore, both patients had normal fecal 51Cr-labeled albumin tests, thus excluding excessive gastrointestinal protein loss. We propose that these siblings have a previously unrecognized familial disorder characterized by reduced serum concentrations of IgG and albumin caused by a defect in endogenous catabolism, leading to a short survival of these proteins that is associated in this family with chemical diabetes and a skeletal deformity.