15-C isomers of rapamycin and of 42-C Epi rapamycin, and pharmaceutically acceptable salts or prodrugs thereof are disclosed. These compounds are potential immunomodulatory and anti-inflammatory agents, and may be useful in the treatment of restenosis and immune and autoimmune diseases. Also disclosed are cancer-, fungal growth-, restenosis-, post-transplant tissue rejection- and immune- and autoimmune disease-inhibiting compositions comprising the invented isomers, and methods of inhibiting cancer, fungal growth, restenosois, post-transplant tissue rejection, and immune and autoimmune disease in a mammal. One particular preferred application of these novel rapamycin derivatives are in implantable medicated devices wherein the prolonged presence of these compound locally are essential to the success of drug containing combination devices.