A new approach for the preparation of nanoparticles is presented. The method is based on control of the gelification phenomenon of alginate by calcium ions, and it leads to small particles of a wide range of very well-defined sizes (250-850 nm) depending on the alignate concentration. The particles are formed in a sodium alginate solution by addition of calcium chloride and then poly-L-lysine. The concentrations of sodium alginate and of calcium chloride were lower than those required for gel formation and corresponded to the formation of a pregel state. The size of the particles formed is greatly dependent on the order of addition of calcium and poly-L-lysine to the sodium alginate solution. This phenomenon can be attributed to the difference in the nature of the interactions between calcium and alginate and between poly-L-lysine and alginate. Furthermore, the data indicate that the formation of the particles probably occurs during the addition of the first component to the sodium alginate solution. Evaluation of the drug-loading capacity was done with doxorubicin as a drug model. The results indicate that alginate nanoparticles are interesting carriers because the drug-loading capacity could be > 50 mg of doxorubicin per 100 mg of alginate.