Serum IgG subclasses and IgG, IgA and IgM were measured in 47 patients with chronic respiratory tract infection, 53 patients with recurrent acute respiratory infections and 66 healthy normal adults. Compared to the healthy control group, mean serum IgG2 was significantly lower in patients with chronic infection (P less than 0.01) and in those with recurrent infection (P less than 0.001), whereas mean serum IgG3 was significantly higher (P less than 0.001 for those with chronic and P less than 0.05 for those with recurrent infection). Five patients with recurrent infection and two with chronic infection had serum IgG2 concentrations greater than 3 s.d. below the geometric mean of the control group, and three of these had concomitant IgA deficiency (less than 0.01 g/l). IgG4 was undetectable in five patients with recurrent and four with chronic infection, two of these also had IgG2 deficiency. In 13 of the 14 patients with IgG subclass deficiencies, total IgG was within the normal range. Selective IgA deficiency with normal IgG subclasses was found in five patients. Twenty-seven patients had raised levels of one or more Ig classes or subclasses. We feel that it is important to screen patients with chronic or recurrent respiratory infection for IgG subclass deficiencies in view of the reported beneficial response to gamma-globulin therapy.