Abstract Lymphocytes from umbilical cords and peripheral adult blood were transformed with Epstein-Barr virus. Cultures of these transformed cells were studied for cell-associated and secreted immunoglobulins (Ig) and for their stability after long-term culture. The adult cell lines produced intracellular and extracellular Ig of all five classes but there was a paucity of IgA in cord cell cultures. Cells producing cytoplasmic Ig were at least 10 times more numerous than those secreting Ig. Both cord and adult lines appeared relatively stable after months of culture indicating that both may be useful in the preparation of immunological reagents.