Fifty pneumonic sheep lungs were examined bacteriologically. Staphylococcus aureus, Pasteurella haemolytica, a-hemolytic Streptococcus species, Actinomyces pyogenes and Escherichia coli bacteria were isolated from 20, 18, 8, 3 and 1 cases, respectively. Among them 8, 11, 3, 3 and 1 isolates were determined as producers for the hydroxymate siderophore, respectively. Sixteen, 15 and 25 isolates were resistant to the bactericidal effects of the horse, cattle and sheep sera, respectively, their count increased exponentially within a range of 1.16-3.41 logs. The obtained results indicate the importance of determination of serum resistance as a bacterial virulence factor in sheep pneumonia