The influence of L-homoarginine on the heat-induced aggregation of three model proteins, i.e. porcine, mink, and human growth hormones was investigated by circular dichroism spectroscopy. It was found that the effect of L-homoarginine as an analogue of arginine depends on the concentration of the additive as well as the protein itself. L-Homoarginine increased the onset temperature of heat-induced aggregation of both porcine and mink growth hormones. However, the formation of human growth hormone aggregates was increased at low concentrations of L-homoarginine. Only at higher concentrations of the additive was the onset temperature of human growth hormone aggregation found to increase. Additional experiments of human growth hormone melting in the presence of histidine, lysine, and sodium chloride were performed. The effect of lysine was similar as in the presence of L-homoarginine. It follows that in protein formulations low concentrations of amino acids should be used with some precaution. At low concentration of additive, depending on the charge of both protein and amino acid used, the promotion of aggregation of unfolding intermediates may occur. (C) 2015 American Institute of Chemical Engineers