Alpha-fetoprotein serum concentrations were determined by radioimmunoassay in 5 patients with echinococcus involvement of the liver. In 4 cases a partial hepatectomy was possible, whereas in 1 case laparotomy had to be confined to an exposure of the liver hilus because of too many dispersed echinococcus cysts. All patients showed normal AFP levels below 20ng/ml before operation. Within the first to third postoperative day maximal AFP increases of AFP level between 2760 and 5600 ng/ml developed, which returned to normal during the following days and weeks. One patient who was submitted to strumectomy and to the same anaesthetic liver charge had no change in her normal AFP concentration. The postoperative changes of AFP level observed are interpreted as an expression of a quick and intense liver cell regeneration following the surgical treatment.