The antibody-mediated immune response (AMIR) of dogs to measles and canine distemper viruses has been described. However, there is little information on the cell-mediated immune response (CMIR). The AMIR and the CMIR of dogs to canine distemper and to measles were examined. The CMIR was determined for 6 weeks by measuring the 3H-thymidine uptake by immune lymphocytes in the lymphocyte transformation test. Concurrently, canine distemper and measles virus serum-neutralization antibodies were measured by a microtitration serum-neutralization test. Dogs vaccinated with canine distemper virus had a CMIR and an AMIR to canine distemper. However, measles virus-vaccinated dogs had only a CMIR to canine distemper. A CMIR in the absence of an AMIR indicates that cell-mediated immunity is the most important immune mechanism in protecting measles virus-vaccinated dogs against canine distemper. Development of CMIR and AMIR to canine distemper and measles antigens depended on the age of the dog at the time of vaccination. Adult and juvenile dogs had immune responses to both canine distemper and measles. Neither virus, however, elicited an immune response in neonates.