Antigen-presenting capacity by monocytes and AMs was determined in 13 patients with sarcoidosis and nine healthy control subjects, using PPD as the antigen. The patients and healthy control subjects all had positive PPD skin tests. Monocytes from both the control subjects and the patients with sarcoidosis exhibited antigen-presenting capacity to autologous peripheral T-lymphocytes, without any significant difference between the two groups. The AMs from patients, but not control subjects, demonstrated antigen-presenting capacity to autologous peripheral T-lymphocytes. Antigen-presenting capacity by monocytes and AMs to lung T-lymphocytes was lower than to peripheral T-lymphocytes, but not significantly. Antigen-presenting capacity was not significantly different between patients with sarcoidosis who had positive and negative PPD skin tests. The mechanism of enhanced antigen-presenting capacity by AMs in sarcoidosis is uncertain at present, but no significant difference was observed in DR antigen expression on AMs between controls and patients with sarcoidosis, and the addition of exogenous IL-1 or IFN-gamma did not induce antigen-presenting capacity by AMs in controls, suggesting that neither increased DR antigen expression on AMs nor increased release of IL-1 or IFN-gamma from AMs is responsible. Thus, these results suggest that T-lymphocyte activation in sarcoidosis may in part be attributable to an enhanced antigen-presenting capacity by AMs.