Abstract: The invention relates to a process for the preparation of a product comprising one or more nanoparticles of calcium phosphate (CaP-NP) with negative surface charge having a ?-potential in the range from ?41.0 mV to ?27.0 mV comprising the steps of: a) maintaining a mixture having a pH in the range from 7 to 10 and comprising an aqueous solution of calcium, an aqueous solution of phosphate and a solution of citrate ions at a temperature in the range from 20° C. to 40° C. for a time in the range from 30 seconds to 10 minutes; b) removing non-reacted ions from the solution of step a), thus obtaining a suspension of one or more nanoparticles of calcium phosphate (CaP-NP); c) recovering the product of one or more nanoparticles of calcium phosphate (CaP-NP) from the suspension of step b). In an advantageous embodiment, the process of the invention provides, in the mixture of step a), also an aqueous solution of one or more therapeutic/diagnostic compounds.