Studies of hairy cell leukemia have yielded conflicting data about the cell of origin in this disease. To investigate this issue, we have examined the state of immunoglobulin genes in the cells of 11 randomly selected spleens showing histologic involvement with hairy cell leukemia. DNA was extracted from splenic tissue samples and digested with restriction endonucleases. Following agarose gel electrophoresis and transfer to nitrocellulose filters or activated nylon membranes, splenic DNA was hybridized with radiolabeled DNA fragment probes specific for the constant regions of the immunoglobulin heavy chain and kappa and lambda light chain genes. Autoradiograms of the hybridized DNA in each case revealed rearrangements of a heavy chain gene and at least one light chain gene. In addition, immunophenotyping of cellular immunoglobulin polypeptides was carried out on frozen tissue sections from all but one case. In each case in which an immunoglobulin polypeptide could be detected, a rearrangement was present in the DNA of the corresponding immunoglobulin gene. These studies offer strong evidence for endogenous immunoglobulin synthesis in hairy cells and for the B lymphocytic character of this leukemia.