Forty seven positive clinical mastitis milk samples of Streptococcus spp. were obtained from cows in and around Ranchi. They were treated in vitro with levofloxacin, ceftriaxone, amikacin and ampicillin and thereafter a smear was prepared, dried and subjected to acridine orange staining. Quantitative analysis of the effects of different antibacterial agents on bacterial phagocytosis and intracellular killing and degradation was done by acridine orange fluorescence technique. The present study revealed that ceftriaxone and amikacin proved to be better for parentral application due to their higher bactericidal and poor leucocidal property. Levofloxacin is a better choice for intra mammary use because of its better leucocidal and bactericidal property.