The conjugate of N4-(4-carboxybutyryl)-1-beta-D-arabinofuranosylcytosine (glu-ara-C) with chitosan, named chi-glu-ara-C, was prepared and evaluated on its utility as a macromolecular prodrug. 1-beta-D-Arabinofuranosylcytosine (ara-C) was predominantly regenerated gradually from chi-glu-ara-C in 1/15 M phosphate buffer, pH 7.4, at 37 degrees C, and its amount after 7 d incubation was 56% (w/w). The antitumor effect was evaluated by an increase in the lifespan (ILS) of the mice bearing P388 leukemia at the single administration at 24 h after inoculation. Ara-C exhibited an ILS of 3.4% at a dose of 100 mg/kg, and the survival time was not significantly different from that of the control. However, chi-glu-ara-C showed an ILS of 60.7% at a dose of 88 mg eq ara-C/kg, and the survival time was significantly different from that of the control. Chitosan and chi-glu-ara-C were observed to cause the side effects judged from the body weight loss to a certain extent. However, chi-glu-ara-C was recognized to be useful because of its effectiveness.