This paper deals with the immobilization of alkaline phosphatase by physical gel entrapment within colloidal particles produced by inverse microemulsion polymerization. This approach has been applied first to the production of polyacrylamide nanoparticles and then extended to the preparation of biodegradable latexes based on a polymerizable derivative of dextran. Conditions for production and purification of nanoparticles have been defined in close relation to protein lability. The catalytic activity of the immobilized enzyme has been compared to the enzyme properties in solution.