We have analyzed the pattern of methylation of rearranged and germ line C kappa genes in DNA samples from human B-type chronic lymphatic leukemia lymphocytes and normal fibroblasts, and from granulocytes, T-, and non-T-lymphocytes separated from normal blood. C kappa alleles are flanked by one HpaII site on each side. Leukemic B-CLL DNA from five patients showed demethylation of these two sites in the productively rearranged allele and methylation of both sites in the nonexpressed allele whether rearranged or in germ-line configuration. Non-T-lymphocytes from normal individuals also showed part of the C kappa genes to have demethylation of both HpaII sites. On the other hand, C kappa genes from normal granulocytes, T-lymphocytes, and four of five fibroblast samples were methylated in one or both HpaII sites. We propose that in B-lymphocytes, demethylation of C kappa alleles is a specific event taking place in the expressed genes and being associated with an increased transcriptional activity of the gene. In addition to this specific demethylation, there is also an unspecific one that may appear in cells with silent C kappa genes and that do not modify the expression of the gene.