Production of immunoglobulins or immunoglobulin-like substances in cell culture lines of human leukemia and lymphoma has been demonstrated by radioiodine labeled antibody, fluorescent antibody and immunodiffusion techniques. A direct reaction of a cell smear with radioiodine labeled antibody reagents specific for particular chains of immunoglobulin indicated that all the leukemia and lymphoma cell lines tested produce one or more classes of immunoglobulins. Immunoglobulin-producing activity was shown to vary during culture with respect to the proportion of cells producing immunoglobulin. There is some evidence that synthesis of heavy and light chains proceeds at different rates in these cell lines. Immunodiffusion studies of cell extracts revealed that the proteins produced by these cells were not distinguished antigenically with immunoglobulins of normal serum when tested with anti-sera specific to heavy and light chains.