The growth promoting activity of human growth hormone was enhanced at least 4 to 5-fold by controlled digestion with a bacterial proteinase. The increased growth activity was measured in rats by both the tibial line and weight gain assays. By radioimmunoassay the potentiated material was indistinguishable from intact human growth hormone. Pigeon crop sac stimulating activity was increased 4 to 10-fold by the enzymic modification. Three forms denoted as I, II and III were formed during the digestion. These were separated by chromatography on DEAE-cellulose and all were similar in enhanced biological potencies. Peptide mapping indicated the three forms lacked residues 138–147 but other structural differences could not be determined by this technique. The activation process involved, therefore, conversion of the hormone from a single chain to a double chain-structure.