Abstract: A mammal organ, for example the thymus, or a cell culture is comminuted to form a slurry, accompanied by cell disintegration, high molecular weight substances are precipitated by heating and removed by centrifuging, and residual substances of molecular weight greater than 10,000 are separated out by ultrafiltration; finally, the salts are substantially removed by electrodialysis. A biologically active, low-salt, sterile, pyrogen-free and antigen-free complete extract is obtained and, in contrast to all known processes, no foreign substance or auxiliary, other than water--not even a preservative or a carrier for separation methods--is used.