Efficient introduction and expression of exogenous genes into primary B cells is very important to study B-cell biology and is essential for gene therapy. These efforts have often been impeded by the lack of availability of a simple culture condition for growth and proliferation of primary B cells as well as the lack of vehicles for efficient introduction of genes of interest. In this communication, we have developed a culture condition that supports the growth of primary B cells from beta-gal-immunized mouse spleen for 30 days without the aid of feeder layers. During this period, B cells secreted polyclonal antibodies into the medium. To study expression of an exogenous reporter gene, human growth hormone (hGH) was introduced into cultured B cells using retroviral vectors. hGH was expressed up to 21 days in the absence of drug selection and the infected cells continued to secrete immunoglobulins into the medium.