Serum from patients with carcinoma of the prostate were observed to possess a factor(s) capable of inhibiting the migration of allogeneic leukocytes in the direct leukocyte migration test. Maximum inhibitory activity was obtained with those sera possessing an elevated level of an as yet unidentified factor(s) in the alpha2-globulin fraction of the serum of these patients. The presence of such a factor(s) may prevent interaction between leukocyte-inhibiting factor sensitive cells and tumour cells and emphasizes the role ofhumoral inhibitory or 'immunoregulatory' factors, such as alpha2-globulin, on immunologicresponses as blastogenesis and inhibition of leukocyte migration, as potential abrogators of mechanisms of host-resistance, e.g., immunologic surveillance.