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1. Immunoglobulins are a complex multigene family of proteins specified by genes encoding variable (V), sometimes diversity (D), joining (J), and constant (C) domains. 2. Cross-reactions involving conformational determinants related to the VHa system of rabbits occur on heavy chains of vertebrate species ranging from elasmobranchs to man. 3. Serological markers characteristic of mu chains, the heavy chain of the IgM macroglobulins, occur on homologous heavy chains of species representing all vertebrate classes. 4. Serological markers characteristic of gamma type heavy chains, the major isotype in man, are restricted to the mammals, but are found on representatives of even the most primitive mammals, the egg-laying monotremes. 5. Variable region markers characteristic of lambda light chains are shared by light chains of shark and man. 6. Certain idiotypic markers defined by combining site V region sequences are broadly distributed in evolution. 7. Use of synthetic peptides as antigens and in epitope mapping show that amino acid sequences from the third framework region of the variable domain are broadly shared among light chain in phylogeny and between light chains and T-cell receptor beta chains. 8. The "switch peptides" linking the V and C domains of light chains and T-cell receptors, specified by the C-terminal portion of the J segment and the N-terminus of the constant region, are exposed in the three-dimensional structure of immunoglobulin or Tcrs, show striking homology, and form broadly shared antigenic determinants characteristic of immunoglobulins. 9. Although the multigene nature of the immunoglobulins and the complexity of antigenic determinants expressed by these large proteins renders comparison among molecules difficult, serum immunoglobulins and the closely related T-cell receptors express numerous shared determinants defined on the basis of amino acid sequence homology and three-dimensional conformations.