The effect of 5-Azacytidine (5-AzaC) on melanization was examined in two sublines of the Smyth line (SL) chicken model for autoimmune vitiligo, in two MHC-matched vitiligo-susceptible but normally pigmented controls (BL), and in nonsusceptible controls (LBL). 5-AzaC was administered ip every 3 days from day of hatch to 18 weeks at levels of 1 or 3 mg/kg body wt. Both treatments increased (P < 0.01) the incidence of autoimmune vitiligo in BL controls. In contrast, treatment significantly depressed (P < 0.01) the expected high incidence of vitiligo in one SL subline, but not in the other. There were no apparent pigmentation changes in 5-AzaC-treated LBL controls. 5-AzaC had dose-dependent depressing effects (P < 0.01) on body and lymphoid organ weights. Histological and mitogen assay data suggest negative effects on lymphocyte number and function. The data show that 5-AzaC can initiate autoimmune disease in genetically susceptible but phenotypically normal individuals.