The blood monocyte and lymphocyte populations were studied in 18 patients with acute sarcoidosis and 10 normal subjects. The patients with sarcoidosis were divided in two groups according to their tuberculin sensitivity. Anergic patients (61%) showed fewer lymphocytes, particularly T lymphocytes, relatively more monocytes and an impaired response to PHA. Patients with positive tuberculin test (39%) showed similar results to controls. It is suggested that the impairment of PHA response might be due to the striking increase of monocytes by a diluting effect in cell cultures or by a suppressor effect.