Circulating immune complexes associated Loa loa antigen in sera of 27 subjects with Loa loa (13 'occult' and 14 microfilaraemic infections) was detected with Polyethylene glycol enzyme linked immunosorbent assay (PEG-ELISA) in a controlled study. Loa loa antigen was detected in all 27 sera. There was no correlation between antigen levels and microfilaraemia density. Cross reaction with Mansonella perstans was found. Although this serological test is able to detect all Loa loa infections, its use is limited by cross reaction with other filariasis. It is hoped that sensitivity will be improved by using monoclonal antibodies.