The structural features of immunoglobulins are described. This family of related proteins shows a common structural design: two identical light chains and two identical heavy chains are present in each molecule. Amino acid sequence studies have shown that light chains have one of two types of sequences in the C terminal half, whereas they differ one from the other in the N terminal half. The two halves of the sequence have been designated accordingly variable and common half. Similarly, the heavy chains have a common sequence in the C terminal three-quarters of the sequence and a variable one in the N terminal quarter. Genetic studies on the inheritance of immunoglobulin alleles have been carried out in some mammalian species. The genetic control of immunoglobulin synthesis is reviewed in man, mouse and rabbit. These studies have shown that each allele controls the inheritance of a specific common region, with the exception of one genetic system which seems to control the synthesis of the variable region of rabbit heavy chains. Immunoglobulin chains are clearly synthesized under the control of two distinct genetic elements, one of which specifies the variable region and the other the common region. The possible significance of this type of genetic control of immunoglobulin structure is discussed. It has not yet been established whether each variant of the variable region is coded for by an individual structural gene present in the genome of each individual or whether few genes for variable regions exist, which in the course of the differentiation of lymphoid cells are subject to somatic mutation processes, which generate variability. These two possibilities are discussed and elements in favour of one or the other theory are presented.