Summary Three overt human infections with Kyasanur Forest disease (KFD) virus directly related to laboratory contact are described. Each case was proved serologically, and in two KFD virus was isolated from blood. The epidemiologic association with laboratory exposure is described, and complete immunologic investigation of all potentially infected persons was made to determine the etiology of observed disease. Differences in serologic response to active disease, modified disease, and subclinical infection are presented. This study provides additional interpretable information concerning the significance of heterologous antibodies to subsequent infection with other Group B arthropod borne viruses.