Human monocytes were cultured at 37 degrees C for 72 h, washed, adjusted for viability and compared to freshly prepared monocytes for stimulation of the autologous mixed lymphocyte reaction (AMLR) and effector function. Pre-incubated monocytes were less potent AMLR stimulators than were freshly prepared cells. Pre-incubated monocytes demonstrated less antibody-dependent tumour killing of CCRF-CEM, less killing of Staphylococci and less spontaneous tumour killing of K-562 than did fresh monocytes. Pre-incubated monocytes produced less prostaglandin E2, demonstrated less surface Ia antigen and were less efficient accessory cells for antigen presentation than were fresh monocytes. AMLR stimulation correlated with monocyte killing (r = 0.95) and PGE2 production (r = 0.98). Thus, monocytes pre-incubated for 3 days are less active effector cells, display less surface Ia antigen and are less potent stimulators of the AMLR than fresh monocytes. Moreover, in this system, monocyte effector activity correlates with ability to stimulate the AMLR.