We coupled bacteriophage T4 with standard (0.16%) or low (0.005%) concentrations of NIP‐azide. The low concentration produced haptenated phage (type B) that was inactivated by the IgG anti‐NIP of some rabbits but not by that of other rabbits similarly immunized. The most active IgG fraction at an anti‐NIP concentration of 100 μg/ml inactivated the type B phage with a titre of 2400, while the same anti‐NIP concentration of the least active IgG fraction from another rabbit had a titre of 0.3. With the standard haptenated phage titres only varied within a 20‐fold range. IgM fractions of a few rabbits tested did not appear to exhibit a great variation with the type B phage. The IgG fractions with high titres against the type B phage bound more NIP‐hapten at low (∼ 10−10M) hapten concentrations than those with a low titre, but this superiority was greatly reduced by an increase of hapten concentration to 10−8M. The same phenomenon could be demonstrated by an inhibition technique, lnactivation of type B phage appeared to detect exclusively antibodies with association constants considerably higher than 108. Its affinity threshold seems to be higher than the threshold of two other methods employed, haemagglutination of haptenated erythrocytes and inactivation of the standar