Abstract: The present invention provides osteogenically active protein preparations comprising a heterodimer of P3 OF 31-34 subunit B and P3 OF 31-34 subunit D, which subunits are linked with at least one disulfide bond and methods for their preparation. The invention further provides cell lines transformed with nucleotide sequences encoding P3 OF 31-34 subunit B and P3 OF 31-34 subunit D and vectors comprising those sequences in operative association with an expression control sequence.