The effect of platinum group metal salts on IgE antibody synthesis in an animal model was studied with respect to the magnitude and kinetics of the response. In outbred Hooded Lister Rats immunized with 10 micrograms ovalbumin with B. pertussis as adjuvant and boosted with 1 microgram in saline an enhanced secondary response to ovalbumin was obtained when certain halide platinum salts were given concurrently with primary immunization. This phenomenon was limited to ammonium tetrachloroplatinate II, ammonium hexachloroplatinate IV and to a lesser extent the cesium trichloronitroplatinate II. Platinum group metal salts not possessing this characteristic were cis-dichlorodiammine plantinum II, tetra-ammineplatinum II chloride, ammonium aquopentachlororhodate III and ammonium tetrachloropalladate II. Kinetic studies show that the time courses of the primary and secondary responses were not altered in the presence of the platinum salts. In rats immunized with ovalbumin without adjuvant there was no detectable antibody and concurrent administration of platinum salts also had no effects. Thus platinum salts whilst acting synergistically with adjuvant to enhance antibody synthesis do not act as adjuvants in their own right.