Forty-two infants, aged 6 to 24 months, infected with Plasmodium falciparum were identified in Kisumu, Kenya. Because of their age, all were presumably not semi-immune to malaria. Each infant was treated with 25 mg/kg chloroquine base and followed for 7 days. Forty-one infections were sensitive to chloroquine in vivo; asexual parasites disappeared in all by day 4 and were not present on days 5, 6, or 7. One infection was resistant in vivo; parasites disappeared by day 3 but recrudesced on day 4. Rieckmann micro in vitro tests for chloroquine were done on the 42 isolates. Interpretable results were found in 25. In vitro resistance was demonstrated in 18 (72%) isolates, including the patient with in vivo resistance; greater than or equal to 99% inhibition of schizont development only occurred in wells containing greater than or equal to 8 pmol chloroquine base (compared with less than or equal to 5.7 pmol/well for known sensitive isolates). This is the first demonstration of in vivo and in vitro chloroquine-resistant P. falciparum in a Kenyan. Comparison of these results with results from other studies carried out in the same area on the same area on older individuals suggests that the immune response may be playing a role in modifying the expression of resistance.