Mumiyo is a natural product found mainly in the high mountain ranges of the Himalayas and in some other regions, too. It has been well known as an expensive remedy for a number of diseases for thousands of years. It disappeared completely from the Western medicine and pharmacology, but was frequently used in the former Soviet Union. The present paper concentrates on physical, chemical and microbiological properties of mumiyo, investigated by means of thermal analysis, flow microcalorimetry, capillary electrophoresis, scanning electron microscopy, determination of its natural radioactivity and conventional bioassays with bacteria and fungi. The paper shows that mumiyo is a complex mixture of effective pharmacological substances that acts as a natural bacteriostatic or even bactericidal agent. (C) 2003 Elsevier B.V. All rights reserved.