Twenty‐two hours after ip administration to male Wistar rats and BALB/c mice, 1,1,1,2‐tetrachloroethane (1,1,1,2‐TTCE) is bound covalently to DNA, RNA, and proteins of liver, lung, kidney, and stomach. The in vivo reactivity leads to binding values to DNA generally higher in mouse organs than in rat organs. The covalent binding index (CBI) values (82 in mouse liver DNA and 40 in rat liver DNA) classify 1,1,1,2‐TTCE as a weak to moderate initiator. Both microsomal and cytosolic enzymatic systems from rat and mouse organs are capable of bioactivating 1,1,1,2‐TTCE in vitro. Liver fractions are the most effective. When the activating systems are simultaneously present in the incubation mixture a synergistic effect is observed. Unlike the related chemical 1,1,2,2‐tetrachloroethane (1,1,2,2‐TTCE), which is bioactivated only through an oxidative route, 1,1,1,2‐TTCE metabolism is carried on by oxidative and reductive pathways, both dependent on cytochrome P‐450. 1,1,1,2‐TTCE is also bioactivated by microsomal CSH‐transferases from liver and lung. These data further confirm that correlations exist between structure and genotoxic activity of halocompounds.