Thirty-five Friesian calves in one of Gharbia governorate farms, were divided into 3 groups: first group 15 healthy pre-ruminant, second group 15 healthy ruminant and third group 5 diseased pre-ruminant calves. First and second groups were injected with once of marbofloxacin at a dose of 2 mg/kg b.w. either by i.m., s.c. or i.v. route (5 calves for eash route of injection), while 3rd group was injected with once by i.m. route at the same dose. Blood samples were collected from each calf just prior to the drug injection. 1/2, 4, 8, 12, 16, 20, and 24h post injection, for determination of marbofloxacin concentration by HPLC with UV detector. Marbofloxacin concentrations in the plasma of healthy pre-ruminant calves at 4th after i.m. s.c. and i.v. injection were 0.98, 0.85 and 0.95 mg/ml respectively, whereas, in healthy ruminant calves it were 0.82, 0.80 and 0.66mg/ml respectively at the same time. After 12h it were 0.55, 0.48 and 0.51mg/ml in healthy pre-ruminant, and 0.18, 0.27 and 0.083mg/ml in healthy ruminant, while after 20h it were 0.34, 0.24 and 0.37mg/ml in healthy pre-ruminant, and 0.060, 0.063 and 0.044mg/ml in healthy ruminant ones after its i.m., s.c. and i.v. injection respectively. The plasma levels of marbofloxacin after its i.m. injection in pre-ruminant calves were 0.55 and 0.66mg/ml after 12h, 0.34 and 0.45mg/ml after 20h, and 0.25 and 0.40mg/ml after 24h in healthy and diseased calves respectively.