Lymphocyte responses to the mitogens phytohemagglutinin P(PHA-P), concanavallin A (Con A), and pokeweed (PWM) were studied in 18 patients with progressive systemic sclerosis (PSS). A subgroup of these patients with multisystem involvement showed a significantly decreased lymphocyte response to both Con A and PWM when compared to normal controls. However those PSS patients with myositis, although having multisystem involvement, had normal lymphocyte response to all three mitogens. PHA-P stumulation was normal in all PSS patients. Antinuclear antibodies, elevated sedimentation rates, and positive latex fixation were noted only in the multisystem disease group with abnormal lymphocyte function.