Human monocytes were rendered cytostatic to the human cell line NHIK 3025 by exposure to lymphokine supernatants (LS) from BCG-stimulated lymphocytes. Exposure to LS for 1, 2 and 4 h induced a considerable cytostatic capacity in the monocytes. However, a stronger cytostatic effect was acquired by exposure to LS for 24 h and 72 h. The phagocytosis of 125I-labelled Candida albicans by LS-activated monocytes was compared with phagocytosis by monocytes treated with control supernatants (CS). The ingestion was increased by short exposure to LS. However, a 72-h exposure to LS induced a decreased ingestion capacity. The capacity of the LS-activated monocytes to digest ingested C. albicans was suppressed. DNA synthesis was increased in the LS-activated monocytes, while protein synthesis was not significantly influenced. The cytostatic capacity of LS-activated monocytes was abolished by culture for 24 h after removal of LS. Following removal of LS or CS with subsequent culture for 4 days, morphological and functional signs of differentiation were less marked in the LS-treated than in the CS-treated cells. The survival was also reduced in the former cells. However, these cells were strongly reactivated by re-exposure to LS.