Seven different immunoglobulin G (IgG) preparations for intravenous use were tested for their capacity to inhibit serotonin-release from washed human platelets induced by IgG coupled with bis-diazobenzidine. Using a reference standard consisting of the top third fraction of an ultracentrifuged gammaglobulin preparation for intramuscular use, two chemically treated preparations were less potent inhibitors than the standard while two preparations, one pH 4 treated and another albumin protected, were better inhibitors. A pepsin treated preparation was devoid of inhibitory capacity, whereas Fc fragments derived from human IgG were extremely efficient. Evidence is discussed that the inhibitory capacity is inversely correlated to the content of oligo- and/or dimeric IgG molecules.