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In preliminary experiments aimed at investigating the effect of covalently cross-linked human myeloma subclass proteins on histamine release from human Ieukocytes, we observed one preparation (designated here lgG-HR) made from pooled, purified immunoglobulin G which consistently released histamine from these cells. Dimers and trimers, but not monomers isolated from columns of Sephadex G-200 and Ultrogel AcA22 following incubation of immunoglobulin C (Nordic Laboratories) with dimethyl suberimidate, released histamine from cells of all donors tested. In contrast, cells from the same donors showed variable responsiveness to dimers of IgE (prepared by similar techniques) or to anti-IgE. lgG-HR failed to release histamine from a "basophil-rich" mononuclear cell preparation depleted of most of the erythrocytes, platelets. neutrophils and eosinophils by centrifugation through a Ficoll-Hypaque cushion. The data suggest that IgG-HR was releasing histamine indirectly from basophils by first interacting with another cell. IgG oligomers prepared from different sources of pooled, purified lgG failed to release histamine. Although we did not have sufficient IgG-HR to adequately define this releasing activity, we feel that the data represent a potentially novel, if rare, mechanism of mediator release involving basophils and another cell. [ABSTRACT FROM AUTHOR]