Peripheral blood lymphocytes (PBL) from Herpesvirus saimiri (HVS) infected owl monkeys lose their ability to respond to PHA during the development of lymphoma. In this study, five HVS-infected owl monkeys were examined for their ability to respond to PHA and to confer this loss of PHA reactivity to normal lymphocytes. Four of the monkeys developed HVS-induced disease and lost their ability to respond to PHA. Although less affected, reduced responses were also found with PWM. The animals which developed disease and suppressed PHA responses were found to be able to confer this loss of PHA reactivity to normal lymphocytes. One HVS-infected monkey did not develop disease or loss of PHA reactivity and did not confer a loss of PHA reactivity to normal lymphocytes. In the four affected monkeys, the conference of suppression to PHA was not accompanied by a concomitant reduction in response to PWM by the normal lymphocytes, demonstrating some specificity in the suppressor cell activity. Thus, the presence of suppressor cell activity appears to be associated with the inhibition of T-cell function and possibly the eventual development of lymphoma.