The yeast Saccharomyces cerevisiae X2180-1A (wild) and its mutants X2180-1A-4 (mnn 1) and X2180-1A-5 (mnn 2) defective in mannan biosynthesis were used as enzyme sources to catalyze in vitro mannosyl transfer from GDP-[14C-U]-mannose to endogenous glycoproteins as well as to exogenous, low-molecular weight acceptors. While the enzyme preparation from the wild strain exhibited all mannosyl transferase activities involved in mannan biosynthesis by catalyzing the synthesis of characteristic mannoprotein, the enzyme from mnn 1 mutant failed to catalyze the synthesis of alpha(1 leads to 3) mannoside linkages both with endogenous as well as with exogenous acceptors. The enzyme preparation from the mnn 2 mutant catalyzed the formation of mannoprotein very similar to that obtained with the enzyme from the wild strain. The most important difference was the formation of a higher number of unsubstituted mannosyl units in the alpha(1 leads to 6) linked mannan backbone. The observed results support the hypothesis that in the mnn 1 the mutation has altered the structural gene involved in biosynthesis of an alpha(1 leads to 3) mannosyl transferase catalyzing the addition of alpha(1 leads to 3) linked mannosyl units to alpha(1 leads to 2) linked mannotrioses in the polysaccharide side chains and in the oligosaccharides attached to serine and/or threonine in the protein part of mannan molecule. The mnn 2 mutant represents most probably a kind of regulatory mutation where the activity of an alpha(1 leads to 2) mannosyl transferase adding the mannosyl units directly to alpha(1 leads to 6) linked backbone in the outer region of polysaccharide part of yeast mannan is repressed in vivo but becomes significant in vitro.