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The udders of 100 heifers were examined for clinical changes during parturition. Of each quarter colostral samples were taken and analyzed bacteriologically. 35 heifers (13.5% of the quarters) showed clinical changes of the quarters and/or of colostral samples. 14 of these animals (4.75% of the quarters) suffered from cellulitis-like mastitis. 13 (5.25% of the quarters) had acute catarrhalic mastitis and eight heifers (3.5% of the quarters) showed acute galactophoritis. In all milk samples of animals with pathological changes of the udder or of colostrum the causing bacteria could be identified. Besides the group of 35 heifers with clinical lesions udder-pathogenic bacteria were found in samples of 24 apparently healthy animals (20.5% of the quarters) during parturition. However, these animals failed to show clinical signs of inflammation at least until the end of this study (10 days post partum). Isolated bacteria included facultative and obligate pathogenic species, 70% of which were gram-positive cocci (i.e. Streptococcus [Sc.] uberis, Sc. dysgalactiae, Sc. agalactiae, Staphylococcus aureus and other staphylococci) versus 30% of gram-negative bacteria (i.e. Escherichia [E.] coli, Klebsiella and other coliform species. In-vitro-susceptibility tests showed best effectiveness of Penicillin against Sc., Cloxacillin against staphylococci and Gentamycin and Polymyxin B against coliform bacteria. Possible pathways, leading to bacterial infection of first lactating heifers during parturition, are discussed. Pathways of infection seem to be of lesser importance for the development of apparent, sometimes severe mastitis with coliform bacteria.(ABSTRACT TRUNCATED AT 250 WORDS)