Chitosan is stepwise treated with periodic acid, urea and formaldehyde and then introduced hydroxymethyl groups reacted with tetracycline and carminomycin. The conjugates obtained are fully soluble in dimethylformamide in contrast to the free chitosan. The conjugate of chitosan with tetracycline preserves almost fully the antibacterial activity of the free tetracycline. An enhenced antitumour activity and decreased toxicity of carminomycin are detected after its conjugation with chitosan.