Methyl-alpha-D-mannopyranoside, mannooligosaccharides obtained by acetolysis of yeast mannan, and pure mannans isolated from the cell walls of pathogenic (Candida albicans) and nonpathogenic (Saccharomyces cerevisiae) yeasts were used for treatment of rat adjuvant arthritis. The arthritis was induced by the application of Freund's complete adjuvant into the tail region of rats. The mannose substances were injected into the arthritic rats intraperitonealy at different time intervals. Levels of serum albumin, changes in hindpaws swelling and radiographs were measured in infected rats as variables of the inflammation and destructive arthritic changes. While mannan from C. albicans inhibited both the inflammation and destructive arthritic changes, mannan from S. cerevisiae showed less effect. However, acetolysate of S. cerevisiae mannan as well as simple methyl-alpha-D-mannopyranoside inhibited both inflammation and destructive arthritic changes to a similar degree as mannan isolated from C. albicans. The effect, which is not dose dependent indicates its possible immunoregulatory mechanism. This is the first time a therapeutic effect of simple carbohydrates on rat adjuvant arthritis has been described.