Studies were carried out on the role of micro-organisms in the decomposition of ancient vegetable tanned leather. The samples were neutralized or sterilized to study changes in chemical composition of extracts after their long-term humidification. The extracted (soluble compounds and tannins) leather powder inoculated with various micro-organisms was analyzed after 8 months of microbial development. It was shown that neutralization of leather causes an abundant development of micro-organism accompanied by chemical changes, and that autoclaving does not bring about the accelerated chemical hydrolysis of leather. The most active organisms in the decay were Chaetomium globosum, Scopulariopsis brevicaulis and Streptomyces sp. I. Extensive deterioration of old leather collagen fibres was proved by chemical and SEM analysis. Table, references.