Abstract Figure 1 illustrates schematically a mechanism that my colleagues and I believe may participate in the cooperative effect in antibody formation. Its essential feature is that thymus-derived cells, in response to antigen, release a soluble factor that is cytophilic for local macrophages and there binds antigen for presentation to the antibody-forming cell. Factors that inhibit macrophage migration (MIF) are known to be released in response to antigen from lymphocytes of animals showing delayed hypersensitivity (Bloom & Bennett 1966; David 1966). Delayed hypersensitivity reactions, and as far as is known MIF production, are mediated by thumus derived cells and in the hapten-carrier situation are largely carrier specific (Benacerraf & Levine 1962; Jankovic, Waksman & Arnason 1962; David & Schlossman 1968).