The growth promoters are substances when fed to animals improve animals’ production performance parameters. These growth promoters may be antibiotic growth promoters, probiotic; prebiotic, synbiotics (probiotic microorganisms with prebiotic substances) anThe present work was performed to investigate the comparative effect of probiotic and boldenone undecylenate on growth performance parameters (body weight, weight gain, feed consumption and feed conversion rate). Their effects on blood picture and some blThis research was done on twenty male buffalo calves 7-8 month old and their weights were 164 -170 kg. divided into 4 equall group each of 5 calves.Group 1: Healthy buffalo calves (control)Group 2: Healthy buffalo calves supplemented orally with therapeutic dose of probiotic 1kg/ton feedGroup 3: Healthy buffalo calves intramuscular injected with half therapeutic dose of boldenone undecylenate (0.55/kg b. wt.)Group 4: Healthy buffalo calves intramuscular injected with therapeutic dose of boldenone undecylenate( 1.1mg /kg b.wt.)Two blood samples were collected 6 times at 4 weeks intervals post application of probiotic and boldenone undecylenate from calves of all groups. 1st Sample was taken in tube contain EDTA and used for