Cellulose ester membranes (Millipore) or polytetrafluoroethylene (Mitex) membranes were coated with adherent layers taken from Dexter-type long-term cultures, 4-5, 8 or 12 weeks after initiation of culture. The cultures were established with marrow taken from untreated mice or, in some cases, from mice treated with a single lethal dose (LD10) of carmustine (BCNU) or cyclophosphamide. In the studies using untreated mice, the cultures went for 8 or 12 weeks and in the drug studies, for 4-5 weeks. The 8 and 12 week cultures were reseeded at 4 weeks. The membranes were implanted into the peritoneal cavities of mice for 3-12 months after which they were removed, fixed, sectioned and stained for histologic study. After 6 months of implantation, about 40% of the membranes coated with cells from non-drug-treated mice and 60% of the membranes coated with cells from drug-treated mice contained hematopoietic elements; often there were foci of trilineal hematopoiesis. Hematopoiesis never occurred without bone formation, but the reverse was not true. Membranes coated with adherent layers established from marrow of mice treated with cyclophosphamide or BCNU showed two main characteristics: 1) they supported hematopoiesis normally, and 2) the regeneration of stroma and hematopoiesis occurred earlier than in membranes coated with stroma derived from normal mice, perhaps because the cells from the drug-treated mice spent a shorter time in culture. In vitro culture may damage cells required to condition the membrane for hematopoiesis.