The purpose of the study was to determine the etiological mastitis agents in cows from herds located in the western part of Poland in years 2003-2005. Bacteriological examinations of 18,713 samples taken from California Mastitis Test (CMT) positive quarters were performed according to standard methods. Some Gram-negative bacilli and Gram-positive cocci were examined with the use of API tests (Analytical Profile Index). It was found that 32.7% of samples that were CMT positive were culture negative and 9.3% of samples were contaminated. Streptococcus species (15.7%), coagulase negative staphylococci (CNS) (14.6%), Staph. aureus (8.6%), Gram-negative bacilli (4.0%) and Corynebacterium species (3.8%) were most frequently isolated. Escherichia coli (52.3%) dominated among Gram-negative bacilli followed by Klebsiella pneumoniae (4.1%), Pseudomonas aeruginosa (3.6%), Enterobacter cloacae (3.6%), Serratia marcescens (3.1%), Pasteurella multocida (3.1%), Acinetobacter lwofii (3.1%), and 26 other bacteria species. Stahylococcus xylosus (31.5%), Staph. hyicus (12.2%), Staph. haemolyticus (12.2%), Staph. sciuri (11.6%), Staph. chromogenes (8.8%), Staph. epidermidis (8.3%) and Staph. simulans (6.1%) were the most frequent CNS. Streptococcus uberis (50%), Str. dysgalactiae (19.7%), Str. acidominimus (6.6%) and Enterococcus faecalis (5.3%) were mostly found among CAMP-negative streptococci. An increase in frequency of mastitis caused by non-agalactiae streptococci, Staph. aureus, A. pyogenes and yeast-like fungi and a decrease in that produced by Str. agalactiae in 2005 as compared to years 2003 or 2004 were observed.