Eleven patients requiring malaria therapy were inoculated intradermally with graded doses of sporozoites of the Chesson strain of Plasmodium vivax. Estimated doses of 10 sporozoites were given to four patients, of 100 to three patients, of 1,000 to two patients, and of 10,000 to two patients. Parasitaemia was detected in all patients 12 to 17 days after inoculation; fever began on the 14th to 19th days. The results of the trials are compared with earlier work on a temperate strain of P. vivax in which patients given small doses of sporozoites exhibited long prepatent periods of 257 days or more. It is concluded that the differences in the two strains can be explained by the assumption that, in varying proportions, all strains of P. vivax produce two types of sporozoites, one eliciting short prepatent periods (Type I), and the other lying dormant or developing slowly to give rise to long prepatent periods (Type II). The latter type greatly predominates in temperate strains, but not in tropical strains; at high dilutions, therefore, pure suspensions of Type II sporozoites of temperate strains can be prepared. It is thought that relapses of P. vivax are in reality a dalyed parasitaemia arising from Type II sporozoites.