A large proportion of systemic lupus erythematosus (SLE) patients develop glomerulonephritis, coincident with the appearance of autoantibodies to C1q, the Fcrecognizing collagen-like subcomponent of the first component of complement, C1. The MRL/lpr/lpr mouse is an established model for SLE, developing both antinuclear and anti-type II collagen autoantibodies, and rheumatoid factors(s), exhibiting reduced complement levels and later on developing glomerulonephritis and often arthritis. We report here an age-dependent decrease in serum C1q levels coincident with the development of IgG2b autoantibodies reactive with mouse C1q in MRL/lpr/lpr mice. Unlike IgG2b, although high levels of IgM, IgG1 and IgG2a are present in these mice, few, if any, antibodies of these sub-classes reactive with mouse C1q were observed in this study. This is the first report of autoantibodies against autologous C1q in an animal model, and the results should facilitate in clarification of the roles of C1q and autoantibodies reactive with C1q in SLE, as well as their potential connection with glomerulonephritis.