In this study immunoglobulin structural variation was examined through analysis of L-ohain type proteins produced by a variety of distinct plasma cell tumors of the mouse. These proteins were examined by serological techniques, quantitative amino acid analysis and peptide mapping. It appeared from these studies that theso proteins all contained a common amino acid sequence throughout a large portion of the molecule. Nevertheless, each of the proteins contained a unique region of amino acid sequence which allowed it to be distinguished from any other on the basis of certain variable peptides. This immediately suggested the possibility of a second chain, loosely linked by non-covalent bonding or joined through disulfide bonding. To examine this possibility, performic acid oxidation and disulfide reduction procedures were carried out. Following these reactions, several methods of separation failed to demonstrate any alteration in either the poptide map patterns or the amino acid composition of these proteins. It seems, therefore, that if a second chain is present it is covalently linked through some bond other than a disulfide bond. It is apparent from the peptide map data that variation must occur in some type of "specialized area". Although the role of the various chains of immune globulins is not yet clear, we believe that this remarkable type of amino acid sequence variation reflects in part the mechanism by which specificity is conferred.