One-step enzymatic production of D-p-hydroxyphenylglycine (D-HPG) from 5-substituted hydantoin was carried out using a bacterium Agrobacterium sp I-671 which possesses D-hydantoinase and N-carbamoylase. From the inhibition study, it was found that N-carbamoylase was severely inhibited by ammonium ions which is co-produced with D-HPG. In order to increase the conversion yield of D-HPG, simultanious removal of inhibitory by-product from reaction mixture was carried out using the specific adsorbents for ammonium ions. The conversion yield of D-HPG reached about 98% in the presence of adsorbents in 27 hours, while 50% conversion was observed in the absence of adsorbents.