The method is for producing porous calcium deficient hydroxyapatite granules and comprises the following steps: (a) subjecting granules of calcium sulfate anhydrous (CSA), calcium sulfate dihydrate (CSD) or calcium sulfate hemihydrate (CSH), the granules comprising interconnected pores with a minimum mean diameter of 3 microns, to a first incubation in an alkaline aqueous solution having a pH-value of less than 10 and containing PO43− ions； (b) subjecting the granules to a second incubation in an alkaline aqueous solution having a pH-value of more than 10 and optionally PO43− ions； (c) filtering and drying the obtained porous granules of calcium deficient hydroxyapatite； and wherein (d) the sum of the amounts of PO43− ions present in the first and second incubation is at least as large as the amount of SO42− ions of the granules used for step (a). The porous calcium deficient hydroxyapatite granules obtained by the invention may be purposefully used as carriers for biologically active substances in particular for proteins.
