We previously reported that circulating natural thymocytotoxic autoantibody (NTA) and IgG-anti-DNA antibodies were the major serological characteristics of a substrain of SAM, SAM-P/1. We present here a study of ageing in which we further measured and compared various kinds of circulating IgG antibodies including anti-collagen type II, rheumatoid factor (RF). and anti-2,4,-dinitrophenol (DNP), between SAM-P/l and control SAM-R/1 mice. The results showed that age-associated increases in anti-collagen type II antibodies in SAM-P/1 were distinctively higher than those in SAM-R/1 when the mice were over 4 months of age, and the increases were significantly correlated with increases in NTA, anti-DNA antibodies, RF activities and anti-DNP antibodies. Anti-collagen type II antibody activity was not significantly inhibited by preincubating the antibodies with DNA, IgG-Fc and DNP-BSA samples. These findings suggest that antibodies specifically directed against collagen type II can be produced in a background of polyclonal B cell activation, and that these antibodies in association with NTA and anti-DNA antibodies may play a pathogenic role in the development of accelerated senescence in SAM-P/1 mice.