Allergic reaction to an antitumor agent, cis-dichlorodiammineplatinum(II) (DDP) was investigated. A 15-year-old white male with pulmonary metastases from embryonal carcinoma of testis was treated with a combination of DDP, bleomycin, and vinblastine. The dose of DDP varied from 2 to 2.25 mg/kg given i.v. He received 7 doses of DDP in 9 months. An anaphylactic reaction was seen within 3 min of the initiation of i.v. infusion of the 8th dose of DDP. The reaction was due to atopic hypersensitivity, as confirmed by an immediate wheal and flair reaction and increased histamine release from leukocytes with DDP. His serum IgE level was elevated. Neither the presence of chloride nor the amine grouping in DDP was essential for reactivity. The replacement of platinum with palladium abrogated the reactivity. There was no cross-reactivity with 3 other platinum complexes of known antitumor activity (platinum blue, platinum(II) 1,2-diaminocyclohexane malonate, and platinum(II) ethylenediamine malonate). This was also confirmed by the lack of reaction to subsequent i.v. administration of platinum(II) 1,2-diaminocyclohexane malonate (10 mg/kg) in this patient.