SUMMARY The response of lymphocytes from 17 patients with primary, secondary, and tertiary syphilis to phytohaemagglutinin (PHA) and to allogeneic lymphocytes was normal in heterologous serum; however, the responsiveness of cells from some patients with primary and secondary disease was significantly reduced in the presence of autologous serum. As cells from healthy controls invariably responded better to these stimuli in autologous serum, the sera from 81 patients with syphilis were screened for immunosuppressive properties. Sera from 25 primary, 32 secondary, two tertiary, six congenital, and 16
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