Epstein Barr virus was used to transform the B lymphocytes infiltrating metastatic tumor tissue from seven patients with melanoma. In this way it was possible to establish continuously growing B lymphoblastoid cell lines (LCL) derived from the tumor-infiltrating B lymphocytes from each of the seven patients. Antibody production of up to 50 micrograms/ml could be achieved by such cultures, and the lymphoblastoid cells could be cloned readily by limit dilution on a feeder layer of irradiated fetal fibroblasts. Preliminary analysis of the antibodies produced by lymphoblastoid cell lines established from tumors from two of the patients indicated that most were of IgM type and bound to a panel of melanoma cell target cells, as well as to some nonmelanoma tumor cell lines. Cloned LCL were produced from the tumor-infiltrating B cells from one of the patients, and of 100 such clones tested, 9% secreted antibody that bound to autologous tumor cells, and one of these clones produced antibody that appeared to be melanoma specific.