Kikuyu grass (Pennisetum clandestinum) is widely used as a grazing pasture in Africa and, although it is extensively cultivated as a lawn for sports fields and domestic gardens, its allergenicity has never been studied in vitro. Using an extract of Kikuyu grass pollen, polyacrylamide gel electrophoresis, Western blotting and a monoclonal anti-human IgE antibody, the specific IgE binding, in the serum of 160 allergic and non-allergic patients in the Cape Town area, to a Kikuyu grass extract was studied. IgE in the sera of 43/104 known grass-allergic individuals bound to Kikuyu grass on the Western blots. In addition, 4/28 'non-allergic' control subjects were found to have Kikuyu grass-specific IgE. Five different profiles of specific IgE reactivity to Kikuyu grass antigens were observed. In 29/43 patients, IgE bound to two dominant 48 and 70 kD allergens in the Kikuyu extract. Although a degree of cross-reactivity with Bermuda grass (Cynodon dactylis) was found in immunoabsorption studies, the 48 and 70 kD allergens appear to be unique to Kikuyu grass. Exposure of heparinized blood from Kikuyu grass-positive patients to the Kikuyu extract stimulated the release of histamine from their basophils in vitro. Kikuyu grass pollen is thus identified as an important aero-allergen in South Africa.