Micrococcus luteus and Pseudomonas species were isolated from the gonads and intestine of Nile tilapia, Oreochromis niloticus. M. luteus and Ps. species antagonized Aeromonas hydrophila with inhibition zone of 4 and 9 cm diameter, respectively. Both microorganisms were added to artificial basal diet with 30% crude protein to evaluate their efficacy on the growth-performance and survival rate, besides some blood-parameters and chemistry. Two hundred and forty O. niloticus with average body-weight of 2.35 +/- 0.1 g/fish were equally divided into four treatments of triplet replicates. T(1) was given basal diet (control), T(2) was given basal diet with M. luteus, T(3) was given basal diet with Ps. species, T(4) was given basal diet with both M. luteus and Ps. Species. Fishes were fed twice daily at the rate of 3% of their live body-weight, for six days a week during the experimental periods (90 day). Fish were challenged by A. hydrophila (0.3 x 10(7) cells ml(-1)) via intra-peritoneal injection and kept for 14 more days. The best growth rate, feed utilization and survival rate were noticed with T(2). The erythrocyte-count was significantly higher with T(2) than T(3) than the control T(1). The hematocrit-values (HCT) and hemoglobin-content (Hb) were significantly decreased with T(3) and T(4). The highest glucose-level was recorded with T(3). The challenged fish of T(2) showed 25% mortality, while T(1), T(3) and T(4) showed 80-90% mortality. M. luteus enhanced the fish growth and health. It is recommended to use M. luteus as a probiotic in vivo, while Pseudomonas species showed probiotic effects in vitro only.