Two IgEL and one IgGK producing cell lines have been established in vitro from peripheral blood and bone marrow of an E myeloma patient. Morphologic and immunologic studies indicate that cells of the IgE producing lines are derived from the same clone of myeloma cells which grew in vivo. They have remained essentially unaltered with a stable near-diploid karyotype and continuous production of intact IgE molecules during 18 months in culture. The IgGK producing line has all the characteristics ascribed to permanent lymphoblastoid lines and is considered to be of non-malignant lymphoid origin. The process of establishment is described and it is suggested that lymphoblastoid cells have a selective advantage over myeloma cells. Permanent myeloma cell lines can therefore probably be obtained only if cells capable of forming established lymphoblastoid lines are very few or absent in the original biopsy. The establishment of a permanent cell line continuously producing intact molecules of IgE should permit further studies on the biological role of this class of immunoglobulins.