Germfree colostrum-deprived piglets are immunologically "virgin" and extremely susceptible to microbial infection due to lack of passive maternal immunity. They are, however, highly immunologically competent as determined by their excellent immune response to various antigenic stimulation. The results suggest that antigen is the sole inducer of specific antibody formation and support the hypothesis that the initial step in the "true" primary immune response involves priming of multipotential uncommitted immunocompetent cells ("virgin" X cells) to committed monopotent cells (Y cells), and that Y cells proliferate and differentiate into antibody-forming cells (Z cells).