To create immortal mesenchymal cell lines, we transduced primary human bone marrow mesenchymal cells with telomerase reverse transcriptase (TERT). TERT⁺ mesenchymal cells continued to grow for > 2 years; parallel TERT^– cultures underwent senescence after 15 weeks. TERT⁺ mesenchymal cells did not form foci in soft agar, had a normal karyotype and could differentiate into osteoblasts and chondrocytes. Their capacity to support leukaemic lymphoblasts and normal CD34⁺ haematopoietic cells was equal to or greater than that of primary cells; 42 TERT⁺ mesenchymal cell clones varied in their supporting capacity. Immortalized mesenchymal cells offer a promising tool for identifying molecules that regulate human haematopoiesis.