The malaria transmission potential of wild, infective Anopheles from western Kenya was evaluated by determining the number of sporozoites transmitted in vitro by salivation when their mouthparts were inserted into capillary tubes containing either sucrose or blood. With sucrose, 86.6% of 102 infective Anopheles transmitted a geometric mean (GM) of 3.84 sporozoites (range 1-34). With blood, 23.1% of 104 infective Anopheles, tested on the day of collection, transmitted a GM of 2.30 sporozoites (range 1-117). For Anopheles held 5 days postcapture before testing with blood, 53.6% of 56 transmitted a GM of 6.04 sporozoites (range 1-420). Transmitting Anopheles contained significantly more salivary gland sporozoites than non-transmitters. No significant differences were detected between Anopheles gambiae Giles sensu lato and Anopheles funestus Giles in sporozoite transmission by individuals with sporozoites in their salivary glands. Sporozoites were detected microscopically in the salivary duct from heads in 80.3% of 117 infective Anopheles (GM = 11.2, range 1-71). Sporozoite detection in mosquito heads by ELISA was 25% less efficient than microscopic detection. Over 98% of the infective Anopheles transmitted less than twenty-five Over 98% of the infective Anopheles transmitted less than twenty-five sporozoites. Transmitted sporozoites represented only about 3% of the total sporozoites in the salivary glands suggesting that sporozoite transmission may be restricted to sporozoites in the salivary duct at the time of feeding. Results are discussed in relation to anti-sporozoite vaccine development.