Immunofluorescent staining has been used to detect antigenic determinants on the surface of living lymphoid cells of mice. It has been possible to distinguish two distinct populations of peripheral lymphocytes: one population carries the theta isoantigen and is thymus-dependent; the other has naturally-occurring immunoglobulin determinants on the cell surface and appears to be thymus-independent. The relative proportions of these two populations of cells is characteristic for each type of lymphoid tissue and these proportions have been determined for thymus, lymph node, spleen, and thoracic duct cells.