The Plasmodium falciparum rate was determined by microscopical examination of one salivary gland (three lobes) and by enzyme-linked immunosorbent assay (ELISA) of the other salivary gland in each of 1580 Anopheles mosquitoes collected from western Kenya during both the wet and dry seasons. The sporozoite rate in the wet season was much higher than that in the dry season, and the sporozoite rate determined by ELISA was generally lower than that determined by microscopy. The ELISA gave a positive reaction to circumsporozoite protein in some glands whose counterparts did not show the presence of sporozoites by microscopy, thus giving an 'overestimation' of the sporozoite rate. This overestimation was greater in the dry season than in the wet season, and greater in Anopheles gambiae than in An. funestus, but overall it was only 1.2% (19/1580). These results are at variance with reports of other workers, who have shown ELISA overestimation of the sporozoite rate as high as 30%. Our tests indicated that the ELISA sensitivity was 80.6%, its specificity was 98.7%, and its accuracy was 97.5%.