Binding of biotin-labelled native and denatured collagen type II and of aggregated IgG to frozen sections of synovial tissue from patients with rheumatoid arthritis (RA) or juvenile chronic arthritis (JCA) was investigated with the help of an avidin-biotin-peroxidase (ABC) technique. A large number of lymphocyte-like and plasma cell-like cells within the investigated biopsies aggregated IgG, and can be assumed to produce rheumatoid factors. In five out of six cases a smaller number of lymphocyte-like and plasma cell-like cells bound native collagen type II. Denatured collagen type II bound mainly to cells within the synovial lining and to endothelial cells within the inflamed synovial tissues. Binding of denatured but not of native collagen II was abolished by preincubation with rabbit antibodies towards human fibronectin. It is suggested that the method described here, using biotinylated antigens, may be of value for the study of local antibody production via investigations on frozen tissue sections, and that local antibody production against native collagen type II occurs within the inflamed synovial tissues at least in some cases of rheumatoid arthritis and juvenile chronic arthritis.