IgG subclasses in sera from 48 patients with systemic lupus erythematosus (SLE) were examined by enzyme-linked immunosorbent assay (ELISA), and the relation between the level of IgG subclasses and infections was investigated. In patients with SLE, IgG2 and IgG4 decreased when compared to normal subjects. In patients with infections, IgG3 and IgG4 were decreased when compared to patients without infections. When the level of IgG3 was less than 60 microliter/ml and the level of IgG4 was less than 20 microliters/ml, infection was increased, especially mycotic infections. It was shown that the estimation of IgG3 and IgG4 serum levels served as important indices for infection.