Summary: Bacteriophage T4B was reacted with N-carbody DL-alanine anhydride, leading to the covalent attachment of alanine residues to the phage, concomitant with extensive loss of viability. The alanylated survivors could be inactivated with anti-phage serum, but were also inactivated specifically with an antiserum possessing antipolyalanine specificity. By using the alanylated phage and the “decision” technique of Jerne and Avegno (14), concentrations of 10-8 to 10-9 g/ml of anti-polyalanine antibody could be detected.