A critical early event in the pathogenesis of diabetic retinopathy is leukocyte adhesion to the diabetic retinal vasculature. The process is mediated, in part, by intercellular adhesion molecule-1 (ICAM-1) and results in blood-retinal barrier breakdown and capillary nonperfusion. This study evaluated the expression and function of the corresponding ICAM-1-binding leukocyte beta2-integrins in experimental diabetes. Diabetes was induced in Long Evans rats with streptozotocin. The expression of the surface integrin subunits CD11a, CD11b, and CD18 on rat neutrophils isolated from peripheral blood was quantitated with flow cytometry. In vitro neutrophil adhesion was studied using quantitative endothelial cell-neutrophil adhesion assays. The adhesive role of the integrin subunits CD11a, CD11b, and CD18 was tested using specific neutralizing monoclonal antibodies. CD18 bioactivity was blocked in vivo with anti-CD18 F(ab')2 fragments, and the effect on retinal leukocyte adhesion was quantitated with acridine orange leukocyte fluorography. Neutrophil CD11a, CD11b, and CD18 surface integrin levels were 62% (n = 5, P = 0.006), 54% (n = 5, P = 0.045), and 38% (n = 5, P = 0.009) greater in diabetic versus nondiabetic animals, respectively. Seventy-five percent more neutrophils from diabetic versus nondiabetic animals adhered to rat endothelial cell monolayers (n = 6, P = 0.02). Pretreatment of leukocytes with either anti-CD11b or anti-CD18 antibodies lowered the proportion of adherent diabetic neutrophils by 41% (n = 6, P = 0.01 for each treatment), whereas anti-CD11a antibodies had no significant effect (n = 6, P = 0.5). In vivo, systemic administration of anti-CD18 F(ab')2 fragments decreased diabetic retinal leukostasis by 62% (n = 5, P = 0.001). Neutrophils from diabetic animals exhibit higher levels of surface integrin expression and integrin-mediated adhesion. In vivo, CD18 blockade significantly decreases leukostasis in the diabetic retinal microvasculature. Integrin adhesion molecules may serve as therapeutic targets for the treatment and/or prevention of early diabetic retinopathy.