Summary: The ability of normal human serum to neutralize the coliphages T2, T4, T6 or T7 was either abolished or markedly depressed by heat treatment, decomplementation, absorption with zymosan, or removal of divalent cations from the serum. The phage neutralizing activity of normal serum was inhibited by irradiated phage preparations. The inhibition produced was always greater in homologous systems than with heterologous ones. This would indicate that the normally occurring phage neutralizing substances exhibit some degree of serologic specificity.