A superior antitumor agent is provided. A salt of1-(2-deoxy-2-fluoro-4-thio-.beta.-D-arabinofuranosyl)cytosine shows at leastone or more of such characteristics as (1)it has superior antitumor activity, (2) it shows superior crystallinity, (3)it shows highwater solubility, (4) it does not show deliquescent property, (5) it showssuperiorflowability, (6) it shows superior tableting property, (7) it can bemanufactured with lessenvironmental load, and (8) it can be manufactured in a large scale, andtherefore it isuseful as a bulk drug for medicaments.
