Studies were designed to compare the effect of the route of inoculation of sheep erythrocytes (SRBC) on the development of antibody-forming cells (AFC) in the hilar lymph nodes of BDF1 mice. The major observation from these studies was that the immunogenicity of low concentrations of antigen was dependent on the route of inoculation. Intratracheal, intravenous, or footpad inoculation of low numbers of SRBC (10(7)) did not stimulate development of AFC in hilar lymph nodes. However, intraperitoneal inoculation of the same concentration of antigen induced an immunoglobulin M AFC response in the hilar lymph nodes. Moreover, both intraperitoneal and intratracheal inoculation of 10(7) SRBC primed the mice for an anamnestic AFC response to a subsequent intratracheal inoculation of 10(7) SRBC. These findings support the hypothesis that presentation of antigen by the local or systemic routes may induce either or both a primary a secondary immune response in the draining lymph nodes of the lower respiratory tract.