Preparations of humic acids extracted from different soils by various methods and model humus substances obtained synthetically by oxidation of hydroquinone and pyrocatechin are tested for growth inhibition of representative strains of human pathogenic microorganisms using a micro serial dilution technique. Within the concentration range of less than or equal to 2500 micrograms/ml 57 of 81 natural and also the two synthetic humic acids show antimicrobial activity with differing spectra. These substances inhibit St. epidermidis, St. aureus, Str. pyogenes, S. typhimurium, Prot. vulgaris, Ent. cloacae, Ps. aeruginosa and C. albicans, but not Str. faecalis and E. coli. The degree of activity or the sensitivity of test organisms, respectively, amounts to 2500--1250 micrograms/ml predominantly, partially 625--312 micrograms/ml and can reach values of up to 39 micrograms/ml with synthetic hydroquinone humic acid. The spectrum and degree of activity vary according to the origin and extraction mode of the natural humic acids. The in vitro evidence of efficiacy against human pathogenic microorganisms gives a rational basis of therapeutic use of substances of humic acid type in infectious conditions.