The aqueous humor of the eye contains factors that regulate immunological responses within the immunosuppressive ocular microenvironment. Besides TGF-beta, the proteins in the low molecular weight (< 3500 Da) fraction of normal aqueous humor are also immunosuppressive. The low molecular weight fraction of aqueous humor inhibits IFN-gamma production and proliferation of antigen-stimulated lymph node cells. Neuropeptides are one possible family of low molecular weight factors in aqueous humor. Through the utilization of an antigen capturing enzyme-assay, the immunosuppressive neuropeptide alpha-melanocyte stimulating hormone (alpha-MSH) was detected in normal aqueous humor of humans, rabbits, and mice. The mean concentration of alpha-MSH in normal aqueous humor of humans was 20 +/- 3 pM, of rabbits 11 +/- 1 pM, of BALB/c mice 16 +/- 3 pM, and of C57BL/6 mice 14 +/- 3 pM. These physiological concentrations of alpha-MSH inhibited the production of IFN-gamma by antigen-stimulated lymph node cells. In contrast to the low molecular weight fraction, alpha-MSH did not inhibit proliferation. There was a 26% recovery of IFN-gamma production when alpha-MSH was absorbed from the low molecular weight fraction. The results demonstrate neuropeptides to be constitutive components of normal aqueous humor and that factors with the capability of differential regulation of effector T-cell activity may be present within the immunosuppressive ocular microenvironment.