In 1981, 41 schoolchildren from Saradidi, Kenya, infected with Plasmodium falciparum were treated with chloroquine. All 41 infections were sensitive in vivo: parasitaemia cleared by day 3 and remained absent through day 7. All 17 (of 27) isolates successfully tested for chloroquine sensitivity in vitro were sensitive in the Rieckmann macro test: greater than 1% schizont development did not occur at chloroquine concentrations of greater than 1.25 nmol per ml of blood. In 1982 in the same area 20 P. falciparum infections were sensitive in vivo: parasitaemia cleared by day 5 and did not recur through day 7. Two of the 20 isolates were resistant in vitro with persistent schizont development at greater than 1% of control values at chloroquine concentrations of 1.5 and 3.0 nmol/ml in the macro test and 16 and 32 pmol/well in the Rieckmann micro test (compared with inhibition at less than or equal to 1.25 nmol/ml and less than or equal to 5.7 pmol/well, respectively, for sensitive isolates). In a modified 48-hour test, growth of two additional isolates was not inhibited until chloroquine concentration of 0.06 nmol per ml of medium, a pattern intermediate between that observed with known chloroquine-sensitive (less than or equal to 0.03 nmol/ml) and resistant (greater than or equal to 0.1 nmol/ml) P. falciparum isolates. The results demonstrate a changing pattern of the in vitro response of P. falciparum isolates in Saradidi to chloroquine.