A novel enzyme immunoassay (immune complex transfer enzyme immunoassay) for anti-insulin IgG in guinea pig serum is described. Anti-insulin IgG in guinea pig serum was reacted with dinitrophenyl bovine serum albumin-insulin-horseradish peroxidase conjugate, and the complex formed between guinea pig anti-insulin IgG and the conjugate was trapped onto affinity-purified rabbit (anti-dinitrophenyl bovine serum albumin) IgG-coated polystyrene balls in the presence of bovine serum albumin. After eliminating nonspecific guinea pig IgG in test serum, the complex was eluted from the polystyrene balls with dinitrophenyl-L-lysine and transfered to clean polystyrene balls coated with affinity-purified rabbit (anti-guinea pig IgG)IgG. Peroxidase activity bound to the (anti-guinea pig IgG) IgG-coated polystyrene balls was measured by fluorimetry. The detection limit of anti-insulin IgG in guinea pig serum was lowered 1,000 to 10,000-fold as compared with that by the conventional enzyme immunoassays.