MEG-01SSF cells were prepared from MEG-01S by conditioning in a chemically defined, serum-free medium. The cells grew well in the medium and required transferrin for growth. Doubling time was about 20 hours. The cells retained GpIIb/IIIa as a marker of megakaryocytes and transferrin receptors on the cell surface. Some of the cells in the stationary phase of growth produced platelet-like particles, which bore characteristic microtubule rings.