A guinea pig model of mild measles virus infection was established by the intranasal inoculation with Toyoshima strain. The infection was confirmed by the development of both humoral and cell-mediated immunities to measles virus as well as by the demonstration of transient virus growth in the lymphoid tissues. The virus infection caused a transient suppression of delayed hypersensitivity to purified protein derivative (PPD) at both the induction and expression phases, whereas Jones-Mote-type hypersensitivity to ovalbumin developed in a normal fashion. In the virus-infected animals, the suppressed response to the skin reacting factor was observed as well, however in vitro responsiveness of lymphocytes to PPD was not suppressed. On the other hand, transient enhancement of skin reactivity to phytohemagglutinin by virus infection was noticed. These results may indicate subtle alterations of immune functions in guinea pigs during measles virus infection.