A C1q-ELISA for the detection of immune complexes is described in which the sensitivity was increased by the addition of polyethylene glycol (PEG). Although the ELISA without PEG adequately detected immune complexes in sera from patients with autoimmune disorders, when sera from patients with filariasis were tested, there was little correlation between values obtained with ELISA and the 125I-C1q binding assays. The addition of PEG to the filariasis sera before reacting the bound complexes to the enzyme conjugated anti-IgG increased the sensitivity to allow detection of immune complexes in those sera. This could be done without adversely affecting the reaction with normal sera or sera from patients with systemic lupus erythematosus. The solid phase C1q-ELISA with the PEG modification can be used for the detection of immune complexes in filariasis and should be adaptable for use with sera from other parasitic infections.