Two cases of tetraethyl lead (TEL) poisoning are described. Both subjects had been exposed to TEL in the process of scaling using high pressure water stream during the cleaning work inside the aviation fuel tank. The aviation fuel contains TEL in a concentration of 1. 12 g lead per liter. The affected men failed to wear respirators during the cleaning work because the explosimeter indicated a negative reading for petrol. After one hour of tank cleaning work they suffered from lacrimation, running rhinorrhea and vomiting. Case 1. A 54-year-old man was admitted to a general hospital 3 days after the exposure to TEL and complained of hand tremors, amnesia and disorientation. He was restless, violent and confused in the night. On 12th day after the exposure to TEL, the condition bacame worse with marked agitation, delirium, convulsion, fever and coma. He died on 18th day after the exposure to TEL. During the admission, urinary coproporphyrin and basophilic stippling cells were normal ; no blood and urinary lead determination were done. Case 2. A 48-year-old man, on 2nd day after the exposure to TEL complained of chills, tremors, marked nausea and vomiting which persisted all night. Next morning he was admitted to another hospital. He had generalized tremors, ataxia, disorientation and at night he was suspicious, restless and violent. On 9th day after the exposure to TEL, his insomnia and restlessness gradually improved and he was discharged two months later. In this case, urinary lead determinations were done serially from 20 days to 196 days after the accident and blood lead determination was done once a week. On 20th day after the exposure, blood lead level was 52.3μg/100g, urinary lead concentration 586 μg/l and erythrocyte ALA dehydrase (ALA-D) activity was markedly reduced to 0.11μ mole PBG/ml RBC/hr. On 196th day after the exposure to TEL, his condition was both physically and mentally normal but his blood lead level was slightly elevated to 26. 1 μg/100 g and the urinary lead concentration was still at 37.0μg/l (81μg/24hr). Blood triethyl lead levels were found to be 5.8μg Pb/100g after 56 days, steadily decreasing thereafter to 1.3μg Pb/100 g up to 196 days. In this case, the reactivation of erythrocyte ALA-D and the fall of blood lead levels occurred simultanously in a manner similar to that observed in men exposed to inorganic lead. The regression line for erythrocyte logarithmic ALA-D activities and blood lead levels in this case is identical to that obtained from workers exposed to inorganic lead and the control group occupationally unexposed. These results suggest that the reduced erythrocyte ALA-D activities found in the TEL poisoning was due to inorganic lead resulting from the decomposition of TEL. Workmen handling antiknock additives were investigated regarding potential hazardous effects of tetraalkyl lead (TAL). The subjects consisted of workmen who engaged in mixing TAL into petrol, transportation of TAL by trucks or barges and storage tank cleaing. There were no abnormal values of blood lead levels, erythrocyte ALA-D activities or excreted urinary lead in those workmen.