Inhibitory effects of two fungistatic preparations (A and B) and of propionic acid were tested in a complete feed mixture for broiler chickens (starter mixture). The water content of this mixture was 25.4%, environmental temperature was 24 degrees C and relative air humidity 90%. Propionic acid, which has a high fungistatic effect, served as a standard. The composition of the preparation A was as follows: propionic acid, acetic acid, sorbic acid, citric acid and calcium propionate. The preparation B contained: sorbic acid, citric acid and calcium propionate. Examinations were performed in a naturally contaminated and subsequently sterilized (25 kGy) feed mixture; it was then infected with Aspergillus fumigatus, A. niger, A. parasiticus and Penicillium purpurogenum from the Collection of Animal Pathogenic Microorganisms, Brno. A. flavus, A. fumigatus, A. glaucus, Penicillium sp., Absidia corymbifera, Mucor sp., Rhizomucor pusillus were detected in the naturally contaminated feed mixture. The mycoflora which was found in the native substrate was resistant to both tested fungistatic preparations; and this resulted in mycelium growth from 7th day of incubation. The efficiency of the preparations A,B and of propionic acid in the feed mixture was identical at concentrations of 4 mg/kg, 7 mg/kg and 3 mg/kg, respectively, and their inhibitory effects were lowest at these concentrations (Fig. 2). A high water content in the nutrient substrate resulted in the rapid growth of fungi of the Mucorales species. The relative humidity of the environment (90%) and water content of tested samples affected markedly micromycetes growth in this experiment.(ABSTRACT TRUNCATED AT 250 WORDS)