The present study details the time course and isotype distribution of the immune response to type II collagen in collagen-induced arthritic mice and mice suppressed for collagen-induced arthritis. The serum of arthritic mice was observed to contain significantly higher (p less than 0.005) concentrations of antibodies to type II collagen than that of mice suppressed for arthritis at all times tested. For the arthritic mice anti-type II collagen antibodies ranged from 0.2 +/- 0.2 (SD) to 6.1 +/- 0.7 mg/ml (g/l). Serum values for mice suppressed for arthritis ranged from 0.05 +/- 0.04 to 0.6 +/- 0.04 mg/ml. Analysis of the isotypes of these responses showed an expression of anticollagen molecules restricted to the IgG1 subclass in mice suppressed for collagen arthritis throughout the time course (p less than 0.01). The data indicate that mice suppressed for collagen-induced arthritis can mount a primary and secondary immune response to the arthrogenic stimuli. This response, however, is mainly restricted to the IgG1 subclass of antibodies. This restricted subclass expression of anticollagen antibodies may represent a mechanism of suppression of arthritis in the murine model of collagen-induced arthritis.