Erwinia carotovora L-asparaginase was coupled covalently to colominic acid, a low molecular mass polysialic acid, by reductive amination. Depending on the molar ratios of colominic acid-asparaginase (50:1, 100:1 and 250:1), polysialylated constructs contained 4.2-8.1 molecules of colominic acid per molecule of enzyme. Such constructs retained most (82-86%) of the initial asparaginase activity and also maintained the Km values of the native enzyme towards the substrate asparagine. On exposure to (mouse) blood plasma at 37 degrees C, polysialylated asparaginase constructs exhibited resistance to proteolysis with 65-83% of the initial enzyme activity still present after 6 h. In contrast, most of the native enzyme was inactivated under the same conditions. In vivo experiments with intravenously injected mice revealed a significant increase in the half-life of the polysialylated asparaginase over that observed with the native enzyme. Such an increase was greatest (250%, about 38 h) for the construct with the highest degree of polysialylation. Results suggest that polysialylation of asparaginase and other proteins may provide an alternative means to improve their effective use in therapeutics.