Blastogenic transformation of peripheral lymphocytes with phytohemagglutinin (PHA), conconavalin A (Con A), and pokeweed mitogen (PWM) was studied in 29 patients with rheumatoid arthritis (RA). The PHA response was depressed in a subgroup of RA patients with erosive disease. The Con A response was also depressed and paralleled the PHA response. The PWM response was not depressed. These results lend support to the hypothesis of a functional defect of cellular immunity in RA. It was shown that lymphocyte responsiveness to a single mitogen concentration is not an adequate assessment of the overall responsiveness of the lymphocytes tested.