In vivo platelet activation by circulating immune complexes has been suggested as one of the underlying mechanisms in pre-eclampsia. Using a modification of the polyethylene glycol protein-A immune complex assay, inmune complexes were found in excess of the equivalent of 20 μg/ml heat aggregated IgG in fourteen out of twenty patients diagnosed as having pre-eclampsia. Only six out of nineteen normal controls were found to have similar levels of immune complexes. Furthermore, using a small volume bio-assay method, concentrations of heat aggregated IgG in excess of 20 μg/ml were found to activate platelets to release sufficient concentrations of vasoactive agents to constrict a human blood vessel in vitro. These results support the hypothesis that in vivo platelet activation by immune complexes can release sufficient concentrations of vasoactive agents to contribute to the hypertension characteristic of pre-eclampsia.