Late schizonts from continuous cultures of P. falciparum were concentrated over Percoll, inoculated to various experimental media at the rate of about 20 X 10(6) per 0.5 ml of medium, and incubated in a candle jar at 37 degrees for 1 day. Controls in standard culture medium showed a heavy invasion with young rings in the previously uninfected red cells introduced with the inoculum of schizonts. In a medium of high potassium content containing a 33% extract of human erythrocytes, this invasion was inhibited and many free merozoites were present. If, however, this same medium was supplemented with both ATP, as the dipotassium salt at 1.6 mM, and sodium pyruvate at 3.6 mM, there appeared large numbers of extracellular forms resembling young rings. Examination of these by electron microscopy shows that they are indeed merozoites that have begun to differentiate extracellularly. This suggests that the trigger for differentiation of merozoites may not depend on the process of entry into a red cell but rather on specific factors within the red cell.