A detailed comparison of the primary structure of 15 individual immunoglobulin light chains including 6 human k chains, 2 mouse k chains, and 7 human. chains is given. Their chemical similarities and the possible conformational influence of the invariant residues are discussed. Some preliminary results on heavy chains are also considered. The most striking structural feature common to all L chains examined is the pattern of distribution of nonpolar, but not of polar or ionized residues along the sequences. Both variable (VL) and constant (CL) regions of L chains contain approximately 55 nonpolar residues. Out of these, 30 nonpolar residues are found in all VL, regions at structurally identical sequence sites and 33 in all CL regions. This frequency of consistently nonpolar residues is similar to that in globin chains and is probably due to thermodynamical and stereochemical requirements. The globular folding of L chains is essentially stabilized by hydrophobic interactions and most of the consistently nonpolar sequence positions are likely to represent intrachain hydrophobic contact sites. These, however, require well‐defined distances between the interacting groups. There is no significant similarity between the pattern of nonpolar residues of the VL, and Ch regions. However, the rule of isopolar substitution suggests a similar folding of all VL, (and possibly Vrd regions, on the one hand, and a similar folding of all CL (and possibly CH) regions, on the other. The low a‐helical content of L chains is explained, and some conclusions concerning future structural studies are drawn.