THE variable regions of human immunoglobulin light chains contain three areas of unusually high variability1–4. Similar hypervariable regions have been postulated for human heavy chains5, 6, but there are no amino-acid sequence data to support this idea. These hypervariable regions are particularly interesting because they may be the areas of the immunoglobulin molecule involved in antibody complementarity.We have made use of the recent observation that a variable region subclass of heavy chains is characterized by an unblocked amino-terminal residue7 and of the availability of automated sequencing techniques8, 9 to study this question in detail with additional heavy chain sequences.