The biological activities of bovine growth hormone (bGH) were studied in a transgenic mouse model system. The following experimental design was used: (1) in-vitro mutagenesis of the bGH gene; (2) expression of the mutated gene in cultured mouse cells under transcriptional regulation of the mouse metallothionein I promoter; (3) binding studies of the mutated and wild-type protein to mouse liver membrane preparations; (4) generation of transgenic mice which express the mutant hormone; and (5) growth rate analysis of transgenic mice. Removal of the alanine +1 codon from the bGH gene or a substitution of serine for cysteine 189 does not affect the ability of the mutant protein to influence transgenic mouse growth. Also, mutations which increase the hydrophobicity within the bGH alpha helix 3 region (amino acid residues 109-126) do not alter the enhanced growth rate in transgenic mice which express these mutated bGH proteins.