Immunotherapy in acute arsenic poisoning. We investigated the use of immunotherapy on the treatment of sodium arsenite toxicity. Female balb/c mice injected with arsanilic acid conjugated to a carrier protein (ovalbumin) were shown to produce antibodies (arsenic reactive serum, ARS) reactive with arsanilic acid and sodium arsenite. Serum was tested for anti-ARS antibodies using a solid phase radioimmunoassay. The antisera bound to ARS conjugated to the synthetic copolymer glutamic acid60 tyrosine30 when diluted as high as 1:4096. Following multiple injections of 100 micrograms of arsanilic acid--ovalbumin compound, mortality on injection with sodium arsenite 0.87 mg/kg i.p. one week later decreased to 0 deaths in 22 pretreated mice vs 9 deaths in 29 untreated mice (31% mortality; p less than .005). No decrease in mortality was noted at higher challenges (1.15 mg/kg) of sodium arsenite. Antisera from pretreated mice was injected 0.1 cc i.p. into 12 week old female balb/c mice followed by an injection of sodium arsenite 0.87 mg/kg i.p. at 10 minutes. Again a protective effect was observed with 0 deaths in 18 mice vs eight deaths in 21 mice (38%; p less than .005). Seventeen additional mice were given an injection of 0.87 mg/kg i.p. of sodium arsenite. After 30 minutes, all mice became symptomatic whereupon antisera 0.1 cc i.p. was given. The one day mortality (2/17, 12%) was possibly lower than the combined control mortality (17/50, 34%; p less than 0.07). There was no change in mortality noted when antisera was administered to mice acutely exposed to 5 mg/kg HgCl2. 