Methods for producing secreted biologically active peptide dimers are disclosed. The methods for producing secreted biologically active peptide dimers utilize a DNA sequence encoding a peptide requiring dimerization for biological activity joined to a dimerizing protein. Polypeptides comprising essentially the extracellular domain of a human PDGF receptor fused to dimerizing proteins, the portion being capable of binding human PDGF or an isoform thereof, are also disclosed. The polypeptides may be used within methods for determining the presence of and for purifying human PDGF or isoforms thereof. Pharmaceutical and diagnostic compositions utilizing the polypeptides are also disclosed.