The monocyte/macrophage (Mφ)-dependency for antigen-induced production of the lymphokine, leukocyte migration inhibitory factor (LIF), was investigated using a Mφ pulse-exposure technique. Mφ-depleted, purified T lymphocytes did not elaborate LIF in response to the recall antigen, tuberculin (PPD). Addition of Mφ's pulsed with PPD rectified the response. Exposure of the Mφ's down to 3 min, even at 0 °C, was efficacious. PPD-exposed Mφ's, either killed or rendered incapable of protein synthesis, failed to activate the T cells. However, PPD-exposed, killed Mφ's triggered LIF production if exogenous interleukin 1 (IL-1) was provided. We suggest that Mφ “presentation” of antigen in this test system is a passive, albeit necessary, process; the requirement for Mφ metabolism being confined to the elaboration of IL-1. Judged by the results of kinetic experiments, the latter stimulus appears to be mediated most effectively from 2 to 4 hr after antigenic challenge.