A 23 year old woman developed thrombocytopenic purpura after drinking "vodka and tonic". This was the result of hypersensitivity to the quinine contained in the tonic water. Quinine-induced thrombo-cytopenia is due to the formation of an anti-platelet antibody in susceptible individuals. As with other examples of immunologic drug-induced thrombocytopenia, platelet destruction requires the presence of both drug and antibody. Clinically, a history of prior quinine administration can usually be obtained. Readministration of the drug, even years later, promptly evokes platelet destruction, hemorrhagic bullae of the tongue and oral mucous membranes, and generalized purpura. Cessation of bleeding manifestations and return of the platelet count to normal levels occur as soon as the drug is excreted - usually 3 to 4 days. The small amounts of quinine (8 mgm%) contained in quinine water emphasizes the marked susceptibility of sensitized persons to the drug. Test doses of the drug should, therefore, never be employed. The diagnosis can be confirmed by a simple in vitro test based on the absence of clot retraction in the presence of quinine. In control tubes containing quinidine, normal clot retraction will be seen since no cross reactivity occurs either clinically or in vitro between these 2 optical isomers. Every patient who develops thrombocytopenia should be questioned carefully about exposure to drugs or other chemicals. Occasionally, as in the present case, an unusual source of exposure may be detected.