Increased permeability of the vascular endothelium is a critical part of pathological processes such as inflammation and wound healing. While several cytokines have been shown to increase endothelial permeability, none has been shown to reduce it. We studied the effect of interferon-alpha 2b on the barrier function of bovine retinal capillary endothelial cells in vitro and found that it increased the electrical resistance of the monolayers (treated cells 211 +/- 6 ohm.cm2, untreated 109 +/- 14 ohm.cm2), decreased their permeability to inulin (Pc inulin of untreated cells 0.73 +/- 0.21 x 10(-6) cm/sec, treated 0.31 +/- 0.23 x 10(-6) cm/sec, P = 0.015), and enhanced their morphological differentiation. Interferon-gamma had no effect. No effect of interferon-alpha treatment on the permeability of bovine aortic endothelial cells was found. We conclude that interferon-alpha 2b can enhance the barrier function of retinal microvascular endothelium in vitro. This is consistent with the hypothesis that interferon-alpha is an effector of a mechanism which actively promotes tissue homeostasis and suggests that it might have therapeutic potential in diseases characterized by leakage of the vascular endothelium.