For the purpose of determining the toxicity of mercuric chloride the author performed experiments to find out LD(50) of this compound with the mice given intraperitoneal injection of mercuric chloride solution and also experiments on the antidotal effect of 6.8-dithiooetanoic acid: and obtained the following results. 1. The LD(50) values determined by Van den Waerdens method are as follows: 1 day LD(50)=0.01116±0.00102mg 2 day LD(50)=0.00997±0.001036mg 3 day LD(50)=0.00837±0.00108mg 4 day LD(50)=0.00712±0.00109mg 5 day LD(50)=0.0064±0.00109mg 6 day LD(50)=0.00574±0.00125mg 2. When 6.8-dithiooctanoic acid is injected to mice prior to intraperitoneal injection of the mercuric chloride solution, it has been found that fatality rate and toxic symptoms (adecrease in the erythrocyte count and an increase in the leucocyte count) are somewhat suppressed.