The serum antibody response to bacteriophage phi chi 174 in germ-free and conventionally reared mice. I. Assay of neutralizing antibody by a 50 per cent neutralization method.

The experimental conditions most suitable for use in the assay of serum neutralizing antibody specific for bacteriophage ΦX174 by a 50 per cent neutralization method were investigated. Although assays conducted at 37° more readily revealed the presence of background neutralizing activity in the serum of non-immunized germ-free and conventionally reared mice, assays performed at 4° were more suitable for use in detecting the development of newly synthesized neutralizing antibody. In definitive experiments, the SD values obtained, i.e. the reciprocal of the serum dilution producing 50 per cent neutralization of added bacteriophage, were found to be directly proportional to the concentration of neutralizing antibody present in serum. The magnitude of the SD values obtained suggests that the procedure may be employed advantageously for the detection of very small amounts of antibody and in studies dealing with the kinetics of the antibody response.