The coupling of nystatin (Nys), a water-insoluble antifungal agent, to dextran via an imine or amine bond was systematically investigated. Dextran was first oxidized to dialdehyde dextran using potassium periodate, purified from the oxidizing agent, and reacted with Nys to form the Schiff base. The Schiff base was reduced to the amine using borohydride. All reactions took place in water. The purification of the oxidized dextran from the oxidizing agent was essential to prevent oxidative degradation of Nys at the coupling step. The effects on the coupling yield of the following factors : dextran molecular weight, degree of oxidation (aldehyde content), Nys to dextran ratio, temperature, and reaction pH were studied. A 95% coupling yield was obtained at the optimized coupling conditions : pH 8.9 +/- 0.1, 50% degree of oxidation, and initial ratio of Nys to dialdehyde dextran 1 : 2.5. In all experiments, dextran was decreased in molecular weight during the oxidation step. Both imine and amine forms of Nys-dextran conjugates were soluble in water and exhibited improved stability in aqueous solutions as compared to the unbound drug. The conjugates showed comparable minimum inhibitory concentration (MIC) values against Candida albicans and Cryptococcus neoformans. The conjugates were about 25 times less toxic than free Nys after a single injection in mice.