Two hundred and thirty-one randomly selected schoolchildren and adults from rural Gambian villates showed no evidence of asthma, nor could any case of asthma be found in three rural villages with a total population of over 1200. In contrast, asthmatic patients were identified without difficulty in the capital town. The geometric mean serum IgE level of 131 rural schoolchildren was 962 mu/ml, whereas that of sixty age-matched urban schoolchildren was 368 mj/ml (P smaller than 0-001). Forty-four asthmatics, all from the capital town, had a mean serum IgE level of 405 mu/ml, not significantly different from that of urban schoolchildren. Twenty-one asthmatics showed skin sensitivity to house dust mite antigen, but their IgE levels were not significantly higher than asthmatics without skin sensitivity. The incidence of positive skin tests to mite antigen amongst normal Gambians, both rural and urban, was around 1%. The findings are discussed in relation to the hypothesis that parasite-induced IgE may prevent the development of atopic illness.