This thesis is concerned with the in vitro initiation of a primary immune response to a chemically defined haptenic determinant, trinitrophenyl (TNP). It had been previously shown that coliphage can be used successfully as in vitro immunogens. The first portion of this thesis involves characterizing the in vitro primary anti-hapten immune response to trinitrophenyl-bacteriophage T4 (TNP-T4). Evidence is presented indicating that the anti TNP response to this immunogen is thymus (T-cell) independent.