In vitro and in vivo uptake of soluble Ag-Ab complexes by eosinophilic leukocytes was studied. In the in vitro experiments, soluble complexes containing ragweed antigen and human IgM or IgG antibodies were much more frequently taken up by eosinophils than complexes containing IgA antibodies. Complexes containing IgE antibodies (purified) prepared in extreme and moderate antigen (purified fraction of ragweed extract) excess were taken up more frequently by the eosinophils (4.8 or 4.9%) than complexes containing purified IgG antibodies (1.7 and 0.9%). In vivo experiments were performed on rats using soluble Ag-Ab complexes of rat system (homologous) consisting of ragweed antigen and rat antibodies and complexes of rabbit and human systems (heterologous) consisting of ragweed and corresponding rabbit or human antibodies. Rats injected with homologous complexes showed a definitely higher percentage of eosinophils taking up complexes (14.1%) than rats injected with heterologous complexes of rabbit (2.3%) or human (1.3%) system.