Incompatibility reactions due to hydroxyethyl starch (HES) were observed during 8 out of 10,273 infusions of 500 ml 6% HES (Plasmasteril®). The clinical symptoms ranged from skin reactions to tachycardia, hypotension and shock. In 3 of the 8 patients with incompatibility serum immunoglobulin concentrations were reduced after the anaphylactoid reaction. Specific antibodies against HES were, however, not detected. Serum IgE levels stayed within their normal limits. Positive reactions of the immediate type to intradermal skin tests with different dilutions of Plasmasteril were obtained in five patients.