Depletion of body protein stores by means of low protein diet or plasmapheresis causes greater susceptibility to liver injury by the arsenical compound, mapharsen. Normal dogs can tolerate mapharsen in doses of 0.006 to 0.008 gm. per kilo whereas in protein-depleted dogs doses of 0.002 to 0.0025 gm. per kilo cause liver injury with jaundice. Methionine (2 to 4 gm.) given by mouth 20 to 24 hours prior to administration of mapharsen is protective and the dogs will tolerate 0.0045 gm. per kilo with no icterus developing. Methionine (1 gm.) intravenously just prior to mapharsen injection is not as consistent in effecting protection, but the smaller dose may be responsible.