The distribution and identity of Thy-1+ cells during ontogeny in the rat were examined by immunofluorescent and histologic techniques, and the relative amount of Thy-1 antigen on different cell types was assessed by quantitative absorption. Thy-1+ cells were subsequently studied by dual immunofluorescence for the simultaneous expression of either T or B lymphocyte markers. The results indicate that the Thy-1 antigen is present on hemopoietic precursors and on at least some of their lymphoid (both T and B) and nonlymphoid descendants. In addition, six serologically distinct subpopulations of lymphoid cells were identified, and their position in T and B cell maturational pathways is discussed.

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