Our previous work has shown that phenyl phosphate acts as an exogenous substrate for GDP-mannose:dolichyl phosphate mannosyltransferase in rat liver microsomal fractions to give rise to phenyl phosphate beta-D-mannose, a compound which, unlike Dol-P-Man (dolichyl phosphate beta-D-mannose), cannot act as mannose donor for further mannose-adding reactions in microsomal fractions. The study has now been extended to the action of various aryl phosphates and structurally related compounds on several other glycosyltransferase systems in the microsomal fractions. (1) Examination of the ability of these compounds to accept sugars from various sugar nucleotides indicated that the individual compounds have specificity as sugar acceptors. Thus phenyl phosphate acted as an effective acceptor for both mannose and glucose, whereas benzenephosphonic acid was active only in accepting mannose. p-Nitrophenyl phosphate was a more effective glucose acceptor than phenyl phosphate, but had only 8% of the mannose-accepting activity of phenyl phosphate. (2) Phenyl phosphate had an inhibitory effect on the transfer of mannose form GDP-mannose to lipid-linked oligosaccharides and to glycoproteins in rat liver microsomal fractions. The inhibition depended on the concentration of phenyl phosphate and on the extent of inhibition of Dol-P-Man synthesis. It is proposed that phenyl phosphate has a direct effect on the synthesis of Dol-P-Man and that its inhibition of synthesis of lipid-linked oligosaccharides and glycoproteins could be a consequence of this effect.