ABSTRACT: Stromal type decidual cells recovered from the murine decidua by a mild collagenase dispersion procedure contain immunoregulatory cells whose ultimate precursors may originate from the bone marrow. To explore the familial relationship of these cells with other cells of the immune system, a battery of cell surface markers recognized on lymphomyeloid cells were examined and quantitated at the morphological level on typical stromal type decidual cells of the dispersed CBA mouse decidua at 8–14 days of syngeneic pregnancy, using a sensitive radioautographic technique. Cells were either labeled directly by exposure to 125I-labeled monoclonal antibodies against Thy-1, Mac-1, or Lyt antigens or indirectly by a sequential exposure to monoclonal anti-I-Ak (Ia. 17) or monospecific anti-I-Jk antibodies and 125I-labeled Protein A. Decidual cells were found to be Thy-1± (13–73% positive, the incidence rising with advancing gestation in the decidua basalis), Mac-1 ± (present on 6–11% on day 8 and 17–32% on day 12), I-A−, 1-J−, and Lyt−. Macrophages within the decidua were Thy-1−, Lyt−, Mac-1 +, and I-A± (present on 5–61% of cells, the incidence rising with advancing gestation). These surface properties of decidual cells along with the presence of FcR, an absence of C3R, and the presence of a unique marker Dec-1 reported by us earlier, distinguish them from most other stromal type immunoregulatory cells such as Langerhans cells (I-A+, FcR+, C3R+, Mac-1−, Thy-1−), follicular dendritic cells (I-A+, FcR+, Thy-1−, Mac-1−, Dec-1−), and dendritic reticular cells (I-A+, FcR−, C3R−, Thy-1−, Lyt−, Mac-1). However, a substantial subpopulation of decidual cells share a few properties (1-A−,Thy-1+) with another marrow derived stromal type cell in the epidermis termed as epidermal dendritic cells that remain to be investigated for other surface markers such as Mac-1, Dec-1, and FcR.