The need for increased antibody production by hybridomas has been approached by the addition to cell cultures of different growth factors; in vitro addition of estradiol-17)β (E₂) to human blood lymphocytes increases the accumulation of plasma-blasts and Ig-secreting cells. Four different murine-murine hybridomas secreting different monoclonal antibodies (MAbs) were treated with E₂. Specific antibody concentration was measured by enzyme-linked immunoadsorbent assay (ELISA) in culture supernatants whereas expression of E₂-receptor in the hybridoma cells was determined by polymerase chain reaction (PCR). When E₂ was added as a growth supplement to α-estrogen receptor positive murine-murine hybridomas it enhanced MAb secretion by as much as 255%, in a dose-dependant manner. This effect lasted for as long as the α-estrogen receptor was detected in the hybridoma cells, was inhibited by tamoxifen and was not observed in α-estrogen receptor negative hybridomas. The synthetic estrogen analogue diethylstilbestrol had no effect. Estradiol-17/β should be added to the list of hybridoma-inducing growth factors.