Plasmodium falciparum and Plasmodium knowlesi have been established in continuous culture using the basic method of Trager & Jensen. Various parameters of the culture system have been examined, namely, the gas requirements, serum and red cell requirements, frequency of medium replacement, and a comparison of static and agitated cultures made. The most important factors affecting growth in vitro seem to be the oxygen tension, red cell concentration, the frequency with which old medium is replaced, and the use of appropriate sera. Preliminary results indicate that horse serum may be possible as a replacement for human serum. Initial studies with P. knowlesi indicate that in the course of adapting to culture, parasites may change their antigenic specificity.