The effect of trichloromethyl and trichloromethyl peroxyl free radicals on protein sulfhydryl content was studied using both, model and enzymatic activation systems. In the model system activation of CCl4 to both free radicals was by UVC light and the target protein was either delipidated or undelipidated albumin. Under air, the CCl3O2. radicals were able to significantly decrease the protein SH in both albumin preparations. A small but signficant effect of UVC alone was observed with defatted albumin. No significant decreases in protein sulfhydryl were observed by .CCl3 attack on the defatted albumin. Reaction of CCl3O2. on cysteine SH led to chloroform formation indicating that a H abstraction reaction is involved in the process. UVC light has an own effect on SH group content. Similar results were obtained when the interaction was with undelipidated albumin rather than with cysteine. Their formation was significantly prevented by Trolox 1 mM in incubation mixture. When the CCl3O2. were generated by liver microsomal activation of CCl4 under air, a significant decrease in microsomal protein SH content was observed. NADPH also exerted an effect of its own. These decreasing effects were fully prevented by either Trolox or EDTA addition to incubation mixtures but not by alpha-tocopherol free or as a succinate ester. Incubation mixtures containing nuclear suspensions and NADPH led to a decrease in protein SH content. This decrease was not enhanced further by the presence of CCl4. No effect on the protein SH content was observed when either mitochondrial or cytosolic fractions were employed to attempt activation of CCl4 to .CCl3/CCl3O2. free radicals. The ability of CCl4 derived free radicals to decrease protein SH in liver microsomes could be involved in loss of activity of key SH enzymes of relevance such as microsomal calcium pump. This pump is known to be damaged during CCl4 poisoning. This effect was blamed to initiate alterations in calcium homeostasis later leading to CCl4 induced liver cell death.