The activity of soluble starch synthetase (ADP-glucose: agr-1,4-glucan agr-4-glucosyltransferase) in the non-purified extract from 16 day-old Bomi barley endosperms (Hordeum vulgare L.) was low and the reaction was non-linear when plotted against protein concentration. Starch synthetase was purified by ammonium sulfate precipitation and DEAE-cellulose chromatography and separated into four fractions. In the absence of an added carbohydrate primer two of the four fractions catalized the synthesis of a methanol-precipitable agr-glucan when high concentrations of sodium citrate and bovine serum albumim were added. The rate of agr-glucan synthesis by the unprimed reaction was higher than for the primed reaction. The four enzyme fractions were active with ADP-Glc, but not with UDP-Glc, both in the primed and in the unprimed reaction.