Malaria-refractory and -susceptible strains of the mosquito vector, Anopheles gambiae, differ in their response to negatively-charged Sephadex CM-25 beads. CM-25 beads elicit a much stronger melanization reaction in refractory mosquitoes than in susceptible mosquitoes. Light microscopic and scanning electron microscopic studies documented a progression from early stages with small spots of melanin adhering to CM-25 beads to late stages where spots had grown and coalesced to form a dark dense capsule. This reaction occurred maximally during the first 18 hr after inoculation; female mosquitoes aged 3-5 days showed 48% of beads heavily melanized by 18 hr postinoculation in the refractory strain and 0-10% in the susceptible strain. Female mosquito age and reproductive status strongly affected the ability to melanize beads. All of the beads were completely melanized in the refractory strain on the day immediately following eclosion (Day 0); thereafter these levels decreased steadily until the last time point on Day 7, when only 23% of beads were melanized in this strain. In the susceptible strain, 53% of beads were heavily melanized on Day 0 and 0-10% were melanized at all other times. At Day 1 and 2 after blood feeding, 85 and 88% of beads, respectively, were heavily melanized in refractory females in comparison with control mosquitoes of the same age which heavily melanize 23-58% of the beads. Blood feeding had little effect on the ability to melanize beads in the susceptible strain.