Capsule size of five isolates of Cryptococcus neoformans was controlled by cultivation in media containing varying amounts of sugar. High concentrations of sugar (e.g., 16%) suppressed encapsulation whereas low concentrations (e.g., 1%) allowed maximal encapsulation. Suppression of capsule size was attributed at least in part to the increased osmolarity of the medium because a medium with low sugar concentration but having high osmolarity (by virtue of added sodium chloride) also produced cells having small capsules. The extent of control was more marked with certain of the isolates than with others. Mice were intravenously inoculated with cells of a single isolate cultivated so as to have either small or large capsules, and virulence was measured by comparing death rates. Results indicate that virulence after such an inoculation is a constant characteristic of an isolate and is not affected by size of the capsule of the cells in the inoculum.