Tumor-infiltrating lymphocytes (TIL) and tumor-associated macrophages (TAM) were recovered from 22 patients with primary lung cancer. The TIL did not exhibit any cytolytic activity against various target cells. The average percentage of TIL recovered was 6.7 +/- 1.3% in patients without recurrence, as compared to 3.6 +/- 0.6% in those with recurrence within 2 years after complete resection of lung cancer (P less than 0.05). The average cytostatic activity of TAM was 38.5 +/- 6.8% in patients without recurrence, although it was 25.2 +/- 4.6% in those with recurrence within 2 years after complete resection (P less than 0.1). The activity of macrophage activating factor (MAF) induced by TIL was also higher in patients showing no recurrence. A significant positive correlation was observed between MAF activity induced by TIL and the cytostatic activity of individual TAM. These results suggest that antitumor activity of TAM may be controlled by specifically sensitized TIL through lymphokines.