One theory of the cause of connective tissue capsule formation around silicone mammary prostheses is based on an immunological interaction. In an in vitro pilot study, it is shown that intraluminal cyclosporine A, a potent T-lymphocyte-specific immunosuppressive agent, can diffuse slowly through the outer shell of a standard double-lumen silicone breast implant. Round silicone tissue expanders containing 50 mg of cyclosporine A were implanted subcutaneously in 10 rats. Ten animals served as controls. Evaluation was performed after three months. A significant decrease in collagen capsular thickness of 21.6 +/- 5.4 microns (mean +/- standard deviation was measured histomorphometrically in the treated group compared with 39.6 +/- 8.6 microns in the control group (p less than 0.001).