Molds are common spoilage microorganisms in many types of foods because they can easily grow on different substrates. Bakery products are among them. Molding of bakery products causes economical loss for bakeries and health risk for consumers. Preservatives can slow down this process, but lately they are avoided by consumers, because they give priority to food without additives. That is why scientists are working a lot on finding natural alternatives which could replace preservatives. In this bachelor thesis we tested the effect of addition of food industry by-products (grape pomace from red and white grapes, olive pomace, flaxseed oil cake, pumpkin oil cake, brewer's spent grain and carob flour) to white wheat flour in concentration of 10 % to prevent molding on bread. We did the microbiological examination of the inside of bread samples after baking and the challenge test with four strains of genera Aspergillus and Penicillium, which commonly grow on bread. We inoculated samples with a certain concentration of mold spores and kept track of their growth kinetics during one week. No by-product had an influence on the microbiological quality of the inside of the bread samples after baking. In the challenge test the addition of some by-products inhibited mold growth, some stimulated it. Wheat flours mixed with carob flour and pumpkin oil cake slowed down the growth of all tested molds. The addition of flaxseed cake and olive pomace generally accelerated the growth kinetics of molds.