Preparation of primary cultures of embryo fibroblasts from genetically engineered mouse strains can provide a valuable resource for analyzing the consequences of genetic alterations at the cellular level. Mouse embryo fibroblasts (MEFs) have been particularly useful in cancer research, as they have facilitated the identification of the genetic changes that allow cells to overcome senescence and proliferate indefinitely in culture. The immortalized MEFs can then acquire additional mutations that lead to anchorage-independent growth and the ability to form tumors in mice. Recently we developed an MEF model system for analysis of the role of the tumor suppressor gene DLC1 in cellular transformation (Qian et al., 2012). In this communication we describe a protocol for the isolation of MEFs from day 13.5-day 14.5 mouse embryos. The MEFs obtained by this procedure are suitable for use in biochemical assays and for further genetic manipulations.