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C1q deficiency has been shown to accelerate spontaneous autoimmunity in mice. We studied the time course of activation of monocytes and lymphocytes in autoimmune and non-autoimmune mice in the presence or absence of C1q as a disease accelerator. Autoimmune MRL\Mp. C1qa −\− and non-autoimmune C57BL\6. C1qa −\− mice were analysed at various time points between 6 and 33 weeks of age and compared to strain- and age-matched C1q-sufficient controls. Splenic and peritoneal leucocytes were analysed by flow cytometry and plasma levels of immunoglobulin M (IgM), total IgG, IgG subclasses and IgM autoantibodies were measured. Both C1q-deficient strains had significantly more splenic monocytes than their controls at all time points analysed. In addition, MRL\Mp. C1qa −\− but not C57BL/6. C1qa −\− mice developed splenic hypercellularity starting at about 12–17 weeks old, had signs of accelerated CD4+ T-cell activation and showed a marked increase in splenic plasma cells and total serum IgM levels from about 22 weeks of age. The accelerated CD4+ T-cell activation was not due to a direct inhibitory effect of C1q on T cells. These data show that C1q deficiency causes splenic monocytosis together with accelerated T-cell activation in an autoimmune-prone mouse strain. [ABSTRACT FROM AUTHOR]