An immunoadsorbent based on immobilized C1q has been developed to remove possibly pathogenic immune complexes from plasma deriving from patients suffering from systemic lupus erythematosus or rheumatoid arthritis. Traditional immobilization procedures based on, e.g., cyanogen bromide activation could not be used to produce an efficient adsorbent. However, by using antibodies directed towards C1q as handles for the immobilization of C1q it was possible to make an adsorbent that efficiently bound immune complexes in plasma. The capacity of the C1q anti-C1q adsorbent to bind artificial immune complexes such as aggregated human globulins or immune complexes from various plasma samples was evaluated. Both batch and column experiments were conducted. The typical capacity in batch was about 1 mg immune complexes/ml gel when incubated with patient plasma samples with high titers of immune complexes. Special attention has to be paid to leakage of undesirable components from the adsorbent. It was found that leakage of C1q occurred but it was not more than after covalent immobilization procedures such as cyanogen bromide.