AbstractThe myeloma line X63.2aRI‐16 isolatedin vitroas spontaneous tertiary class switch variant (γ1→ γ2b→ γ2a→ γ2a,γ1) of X63 (IgG1,x) by fluorescence‐activated cell sorting using class‐specific antisera expresses two heavy chains. X63.2aRI‐16 secretes IgG2a,xas does the parental cell line X63.2a‐25, a hybrid molecule containing γ2aand γ1heavy chains and IgG1,x. The chain of the latter protein is the product of a reverse class switch in regard to the embryonic order of the CHgenes. We purified the immunoglobulins of X63.2aRI‐16, isolated the CNBr fragments of both heavy chains and determined the complete amino acid sequence of the VHdomains and of the CHdomains up to the first subclass‐specific residues. The remaining CNBr fragments of the CHdomains were characterized by amino acid analyses. It was found that both heavy chains of the double producer possess identical VHdomains and CHdomains characteristic for the subclasses γ2aand γ1, respectively. The identities of the two VDJ sequences expressed in X63.2aRI‐16 cells suggest that the reverse class switch event to γ1cannot simply be explained on the basis of a deletion model