121 serum samples from 54 patients with Wilson's disease were tested for antibodies against D-penicillamine by indirect immunofluorescence (DASS-system). IgG antibodies were found in 44 serum samples from 16 patients (31% of all patients). The incidence of serum antibodies was higher in patients with side effects during therapy with D-penicillamine (10 of 13 patients 77%) compared to 6 of 39 15% in patients without side effects. The titre of antibodies was higher in patients with side effects. The antibodies bound complement demonstrated by double immunofluorescence. These observations indicate that complement binding IgG antibodies to D-penicillamine are involved in pathogenesis of side effects during therapy with D-penicillamine.