Humoral responses play a prominent role in development of collagen-induced arthritis in the common marmoset. In Experiment I where a semi-purified collagen type II (CII) was used, (a) a transient CII-specific IgM response was followed by (b) a sustained CII-specific IgG response. Even in animal Mi137 this phenomenon was observed after the booster at day 110. In Experiment II where commercial batches of CII were used both against (c), (d) bovine CII and (e), (f) chicken CII, a transient CII-specific IgM response ((c) and (e)) was followed by a sustained CII-specific IgG response ((d) and (f)). PID, post induction day.