Albumin, transferrin, and lipids can replace serum entirely for support of LPS-stimulated murine B lymphocytes in culture. In the presence of these compounds, growth and maturation to IgM and IgG secretion, induced by lipopolysaccharide (LPS), occurs at the same or higher efficiency in serum-free conditions as in conventional serum-containing medium, even at relatively low cell concentrations. In contrast to the rapid disappearance of LPS reactivity in conventional serum-containing medium, responsiveness remains at initial levels in serum-free conditions for 2 days before slowly declining. Overall lymphocyte survival is also markedly prolonged. In the presence of thymus "filler" cells, the serum-free conditions permit growth of every LPS-responsive cell to a clone of Ig-secreting cells at dilutions as low as a single reactive B cell per culture. The results have several important implications. These include the establishment for the first time of transferrin as a requirement for B lymphocyte responses in culture, and the availability now of conditions for the assay isolation of cell products regulating lymphocyte function, free of interference from undefined serum components.