Many embryonic stem (ES) cell culture protocols rely on the use of a monolayer of primary mouse embryonic fibroblasts (MEF feeder cells). MEF cells perform two important roles in stem cell culture: they secrete several important growth factors into the medium, which help maintain pluripotency, and they provide a cellular matrix for ES cells to grow. The EmbryoMax™ range of PMEF mouse feeder cells provides researchers with a convenient solution for ES cell culture by eliminating the need for time consuming feeder cell isolation and preparation. Derived from day 13 mouse embryos, these cells are frozen into individual vials, containing approximately 5-6 x 106 fibroblasts per vial, and are sold as a pack of 5 vials per kit at various passage number (P1-P3). Several varieties are available, including drug resistant, growth arrested, and not mitomycin-C treated mouse feeder cells.