This study was carried out particularly focusing on he antitumor effect in combination with 5-fluorocytosine(5-FC), antifungal agent, and extracellular cytosine deaminase from Chromobacterium violaceum YK 391 against U-937, K-562 and SNU-C4 cells. While the addition of 10$mu extrm{g}$/100 ${mu}ell$ of anticancer agent, 5-fluorouracil(5-FU), to U-937, K-562 and SNU-C4 caused the decrease of proliferation 90%, 75% and 93% respectively, the addition of 20 $mu extrm{g}$/100 ${mu}ell$ of the extracellular cytosine deaminase and 10 $mu extrm{g}$/100 ${mu}ell$ of antifungal agent 5-FC caused the decrease of proliferation 80%, 70% and 90%, respectively. These results, therefore, reveal that this enzyme has the similar clinical effect for considering of adjuvant antitumor effect. From the above results, the treatment of 5-FC and the cytosine deaminase was very effective and showed the possibility to remove side effects which easily occur by the treatment of 5-FU only. An extracellular cytosine deaminase.