The lymphoproliferative response to measles, mumps, and vaccinia virus-infected monolayers measured in seropositive adults by thymidine incorporation demonstrated that only 5% of individuals responded well to measles virus (stimulation index, greater than 5). Possible explanations for this occurrence include a lack of sensitization, active suppression, or failure in long-term stimulation. To distinguish among these possibilities, we studied the responses to measles virus in 22 immunocompetent individuals during early convalescence from natural measles infection. Substantial responses occurred (stimulation index, 7.03), particularly in a smaller group which included those individuals with milder cases of the disease. The level of responsiveness declined over a period of weeks. Responder and nonresponder cell mixing showed no active cellular suppression. These studies indicate that the low responses to measles virus found in late convalescence represent a lack of prolonged stimulation of the cell population measured in this assay.