Compositions, kits and methods are provided for promoting general health or for prevention or treatment of diseases by using novel recombinant fusion proteins of human serum albumin (HSA) and bioactive molecules. The bioactive molecules may be a protein or peptide having a biological function in vitro or in vivo, and preferably, having a therapeutic activity when administered to a human. By fusing the bioactive molecule to HSA, stability of the bioactive molecule in vivo can be improved and the therapeutic index increased due to reduced toxicity and longer-lasting therapeutic effects in vivo. In addition, manufacturing processes are provided for efficient, cost-effective production of these recombinant proteins in yeast.