Intracutaneous immunization of BALB/c mice with purified inactivated Semliki Forest virus resulted in cellular immunity without detectable antibodies. The animals were protected against subcutaneous challenge, from which the challenge virus spreads slowly. After intraperitoneal challenge, which permits a rapid virus spread, the protection was marginal. Stimulation of the intraperitoneal cell population with thioglycolate before challenge resulted in complete protection. The protection could be transferred to normal mice with peripheral lymph node cells, but not with spleen cells. The course of the infection in immunized and normal mice was also studied. Semliki Forest virus does not multiply in peritoneal cells in vivo. In immunized mice part of the challenge virus in the peritoneal cavity was rapidly eliminated and viremia was reduced. After challenge, immunized mice produced less antibody than normal mice.