Growth of Clostridium, originating mainly from silage, may cause serious late blowing defects in semi-hard cheeses during ripening. In the present project, the possibilities were investigated to use anticlostridial non-starter Lactobacillus (mainly Lb. paracasei), isolated from Danish semi-hard cheeses of high quality, as protective adjunct cultures against clostridia activities in silage and cheese. Screening for anticlostridial activity among non-starter Lactobacillus isolates against selected Clostridium strains showed that almost half (44%) of the naturally occurring non-starter Lactobacillus in Danish semi-hard cheeses possessed anticlostridial activities and 10% possessed a broad anticlostridial activity, and these were selected for further investigations. Antagonistic antimicrobial interactions between some of the selected anticlostridial Lactobacillus strains were demonstrated and are of importance when selecting a mixture of the anticlostridial Lactobacillus strains for adjunct cultures to be used in silage and cheese. Heat resistance of nine anticlostridial Lb. paracasei strains was investigated using a submerged-coil apparatus. Seven of the nine strains survived pasteurisation (73¿C, 15 sec) at low numbers in UHT (Ultra High Temperature) whole fat milk, and identity of the strains was successfully confirmed using repetitive-PCR (Polymerase Chain Reaction) analysis. The potential of Lb. paracasei strains to survive pasteurisation of cheese milk was demonstrated. The behaviour of anticlostridial Lactobacillus strains in grass silage was evaluated in model scale using three different mixed adjunct cultures of anticlostridial Lb. paracasei strains. It was shown by using species specific PCR analysis that the added anticlostridial Lb. paracasei were able to grow and survive in grass silage for at least 60 days and thereby have the potential to aid silage fermentation and improve silage quality. Cheese making experiments (Danbo 45+) with anticlostridial Lactobacillus strains in industrial scale were made using a mixed adjunct culture of five anticlostridial Lactobacillus strains. Gross composition was not influenced by addition of the anticlostridial culture. Excessive gas formation, measured as changes in the cheese specific gravity, occurred only in control cheeses without the anticlostridial culture added. Repetitive-PCR analysis was applied for identification of cheese isolates after four weeks of ripening, and in contrast to control cheeses, the experimental cheeses were dominated by the added anticlostridial Lactobacillus strains. From the results of this thesis it can be concluded that anticlostridial non-starter Lactobacillus isolated from semi-hard Danish cheeses of high quality have potential use as protective adjunct cultures against clostridia activities in silage and cheese.