The present experiment was conducted in order to study the preventive and therapeutic effects of 2-mercaptoethylamine (MEA) on lead poisoning. By inducing experimental lead poisoning in mice and rats with lead, the author investigated the effects of MEA on the body weight and survival of mice and on the body weight and the blood picture of rate similarly treated, and taking these effects of MEA as the criteria and comparing them with the effects of Ca-EDTA similarly administered, the author evaluated the effectiveness of MEA. The following are the results. 1. It has been found that the effects vary according to the species of animals as well as to the amount of lead given, and for the change in the body weight of mice it proved to be superior but for the changes in the survival of mice and in the body weight and blood picture of rats it was clearly inferior to those of Ca-EDTA. 2. Generally speaking, the effect of MEA is superior to Ca-EDTA for the first week after the initiation of its administration but when repeatedly given thereafter the effect of Ca-EDTA supercedes that of MEA. 3. Concerning the mechanism of the action of MEA, at present it is still in a theoretical stage and it remains a problem that requires more extensive studies before confirming its mechanism. However, a deduction may be drawn from the findings of the present experiment. Namely, it is assumed that the effect of MEA is due to some biological action other than its action of producing metal complex and its combining power lead in vivo is extremely weaker than that of Ca-EDTA so that it tends to bring about oxidative disintegration of lead-MEA complex in vivo and thus there is rather a possibility of its unfavorable effect.