In T cell-dependent immune responses, class switch recombination occurs in germinal centres. There is now evidence that Hodgkin/Reed-Sternberg cells are derived from germinal centre B cells. Cytokines specifically determine the direction of class switching, i.e the isotype of the new antibodies. We performed restriction analyses and polymerase chain reaction on the immunoglobulin heavy chain loci for five Hodgkin's disease-derived B-cell lines and one Hodgkin's disease-derived T-cell line in order to analyse class switch recombination. In all the B-cell lines, class switch recombination had been targeted to Calpha4 or Calpha1/2. This showed that cell-line precursors had undergone class switching, probably under the influence of TH2 or TH3 cell-derived cytokines. Deletions comprising several constant region genes were observed in cell lines L428, L1236, L591 and KMH2. Karyotype analyses of two of these revealed translocational breakpoints within the immunoglobulin heavy chain gene locus. Our data support the view that a chromosomal instability may occur during class switch recombination in Hodgkin/Reed-Sternberg cells causing chromosomal breaks. Thus, as in other germinal centre B cell-derived lymphomas, the immunoglobulin gene locus may be frequently involved in structural chromosomal aberrations in Hodgkin's disease.