Immunoglobulin-synthesizing cells from the spleen and bursa were fractionated by the 1 × G sedimentation velocity technique and characterized by their ability to synthesize immunoglobulin and by staining with fluorescent anti-light chain chain. Four subpopulations of immunoglobulin-synthesizing cells were identified. In the bursa, slowly sedimenting (S 2.3 mm/hr) and rapidly sedimenting (S greater than 3.5 mm/hr) subpopulations with surface immunoglobulin were present; in the spleen, a slowly sedimenting (S 2.3 mm/hr) subpopulation with surface immunoglobulin and plasma cells (S greater than 3.5 mm/hr) with large concentrations of intracellular immunoglobulin existed. The subpopulations differed most markedly in their ability to synthesize immunoglobulin (cpm Ig synthesized/106 Ig-positive cells); the rates of immunoglobulin synthesis were in the ratio 1:2:1:900. The slowly sedimenting B cells from the spleen and both subpopulations of B cells from the bursa released small amounts of immunoglobulin into the culture media, whereas, the plasma cells released immunoglobulin at a rate as much as 3700 times greater. Bursal B cells could be further distinguished from splenic B cells by a greater amount of DNA synthesis.