Ultrastructural features of mouse bone marrow mononuclears (BMM) labelled with peroxidase-anti-peroxidase (PAP) complex using rabbit anti-mouse brain serum (RAMBS) and rabbit anti-mouse brain serum, absorbed with thymocytes (RAMBST) were investigated. After incubation of cells with RAMBST which have high activity against mouse pluripotent hemopoietic stem cells (CFUs) about 0.6% of BMM were labelled. According to their fine structure these cells were similar to those described earlier as presumptive pluripotent hematopoietic stem cells (PHSC). After incubation with RAMBS 1.9% of BMM were labelled, including cells marked by RAMBST and typical small lymphocytes with ultrastructure of T-lymphocytes.