Chikungunya virus and dengue type 2 virus were isolated from a single blood specimen taken from a patient in the acute phase of a dengue-like illness seen at Christian Medical College Hospital, Vellore, South India, in October 1964. In serial blood specimens collected from this patient there was an increase in antibody to these same two viruses. The technique for unmasking an agent (such as dengue) with a long incubation period in mice in the presence of an agent with a short incubation period is described.