Monocytopoiesis was analyzed in patients with severe, acute inflammations induced by surgical interventions as well as in others with mild, chronic inflammations in connection with gastric or duodenal ulcers. The state of acute inflammation was assumed to be associated with a high and steeply rising monocyte demand as opposed to the constant and relatively small monocyte recruitment in chronic inflammation. In chronic mild inflammatory reactions DNA synthesis activity of promonocytes was increased by a factor of about two; the promonocyte pool was normal. In patients who underwent surgical operations changes in the following parameters were observed during the first 15 h after start of surgery: (1) average increase in 3H-TDR labelling index by 38%; (2) average enlargement of promonocyte pool by 34%, (3) and relase of immature cells from the bone marrow into the blood. Increase in DNA synthesis activity as well as expansion of the promonocyte pool cause an enhanced monocyte production rate. The ‘shift to the left’ in monocyte egress is equivalent to a reduced stem-cell-to-blood transit time. These variations permit short-term adaptation of monocytopoiesis to varying demands.