The tolerance of peripheral veins to intravenous infusions was evaluated. Of 83 infusions studied, 67 contained amino acids. Phlebitis occurred more commonly with the use of solutions that contained the amino acids. The important factors in the production of phlebitis by amino acid solutions were osmolarity, and the amount of potassium infused per day. Phlebitis was universal when osmolarity exceeded 600 mOsm. Other factors that promoted phlebitis were the presence of antibiotics and the size of the vein.