C1q, a subcomponent of the first complement component, of a 60-year-old female patient with hypocomplementemic vasculitis-urticaria syndrome (HVUS) was characterized. The C1q-precipitin activity (C1q-p) could not be detected by a routine method with 0.6% agarose in 10 mM Na-phosphate buffer containing 10 mM EDTA (pH 7.2). Hemolytic activity of her serum complement (CH50) and levels of C1 and C4 were significantly reduced at the exacerbation stage, but levels of other complement components were almost within the normal range throughout her clinical course. The specific activity of C1q at her exacerbation stage was significantly low, and its elution position on Sephacryl S-300 column was spread toward the low molecular weight in comparison with that of normal plasma. Molecular weights of the delayed fraction of C1q were estimated to be approximately 300,000 on the Sephacryl and 440,000 by the polyacrylamide gel electrophoresis (PAGE) containing sodium dodecyl sulfate (SDS) followed by immunoblotting, respectively. On reduction of her plasma, two bands with molecular weights equivalent to those of B and C chains of the normal C1q in an approximate molar ratio of 2:1 were immunostained. Plasma at her exacerbation stage showed only one precipitation line against anti-human C1q-antiserum which was completely fused with that formed between purified normal human C1q and the same antiserum. The probable structural change of the hypofunctional C1q in the case of this HVUS is discussed.