Relations between the appearance of various components of the immune response were analyzed in two groups of rats sensitized by aerosol and subcutaneous injections in the neck region, respectively. The relations were expressed by Spearman rank correlation coefficient and studied by cluster analysis. In the aerosol-sensitized animals, there was a close association between IgA and IgG antibody levels in bronchial fluid and these in turn were related to IgG levels in serum and more loosely to IgE levels in bronchial fluid. There was an apparent association between IgE antibody formation and mast cell maturation in cultures of regional lymph node cells and the appearance of mucous cells in the lungs. These variables seemed associated with spontaneous cell proliferation in vitro and the numbers of mast cells in the lungs. This indicates that local stimulation with antigen induces local immune responses and immune-mediated migration of cells. In subcutaneously sensitized animals, formation of IgG antibodies in vitro seemed related to the stimulated proliferation of regional lymph node cells. The levels of IgG and IgE antibodies in bronchial fluid and in serum also appeared to be related. Unlike the findings in aerosol-sensitized animals there was no apparent relation between the differentiation of mast cells and mucous cells. This was possibly due to lack of immune-mediated antigen-induced cell migration. The different immune response patterns in aerosol and subcutaneously sensitized rats should be considered when studies are designed aiming to explore the pathogenesis of allergic inflammatory diseases. The findings also indicate that the various parameters of immunity are more closely related in aerosol than in subcutaneously immunized animals.