On the basis of extensive shared idiotypic specificities, two human IgM anti‐gamma‐globulins (Lay/Pom) were selected for complete amino acid sequence analysis of their variable domains. Previous studies on the variable regions of the heavy chains of these proteins had shown but eight amino acid differences, only one of which was within a complementarity‐determining hypervariable region. The complete amino acid sequence of the variable regions of the light chains of these two proteins is the subject of this report. Protein Lay is a typical VχI protein with only five ‘framework’ differences when compared with protein Roy. Protein Pom is best classified as a VχII, but in the ‘framework’ there are 16 differences between it and protein Ti. Although there are extensive differences in the first hypervariable region, the second and third light‐chain hypervariable regions have an identical sequence. The finding of two identical light‐chain and two identical heavy‐chain hypervariable regions in these two proteins, which were selected on the basis of their combining specificities and their idiotypic cross‐reactions, strongly implicates hypervariable regions in the constitution of the idiotypic determinants and the antibody combining site. Additionally, the finding of identical hypervariable regions in light chains of different V‐region subgroups fulfills a prediction of the gene‐interaction concept of antibody variability.