This study was conducted to evaluate the effect of using Opitgen feedadditives as a replacer to soybean meal in the diet of fattening Holistiencalves. Fifteen Holistien calves with an average initial weight 260.26±13.25kgand aged seven month were divided into three groups (five in each) based oninitial body weight and age. Calves were housed under open sheds. Calves inthe first group were fed the control ration (without additives) while the steers inthe second and third groups were fed the control ration with addition 75 or 100gm. of Optigen II (Alltech, Lexington, KY)as a replacer to 0.421 kg of soybeanmeal which is represented of 1.35 % on dry matter basis (Opt. 75)or100 gmOptigen II(Alltech, Lexington, KY)as a replacer to 0.558 kg of soybean mealwhich is represented of 1.84 % on dry matter basis (Opt. 100) for 105 days. Dry matter intake, nutrients apparent digestibility, rumen activity, growthperformance and economic efficiency were determined.The results showed that dry matter intake, organic matter, crudeprotein, ether extract and crude fiber as well as total digestible nutrients anddigestible crude protein were not affected by the different experimental diets.The animals in G2 (Opt.75) had the highest digestibility coefficients of allnutrients followed by animals in G3 (opt. 100), while the control group had thelowest values. The nutritive values expressed as TDN or DCP for animals in G2 (Opt. 75) and G3 (Opt. 100) were slightly higher (P> 0.05) compared withthe control group. There were no significant differences (p>0.05) in pH valueamong the different experimental group during the trial, the pHvalues are within the normal range. But there were significantly (p<0.05) differencesamong the different groups concerning ruminal ammonia-N concentration(NH3-N whereas the G3 (100 g Opt.) recorded the highest NH3-Nconcentration which were 7.13, 8.03 and 9.56 mg/100 ml rumen liquor for G1,G2 and G3, respectively. The concentration of TVF`s in the rumen liquor ofanimals in group II supplemented with optigen (75 g Opt.) was highersignificantly (p<0.05) compared with the other groups. The average daily gainfor G1, G2 and G3 were 1.35, 1.39 and 1.25 kg/day, respectively. Calves in G2 recorded the highest value of average daily gain compared to the othergroups. Also, the animal in G2 (75 g Opt.) recorded the better value of feedconversion compared to the other groups, it improved by 5.21 and 14.04% compared with the control and G3, respectively). The best economicefficiency was recorded by the G2 while the lowest economic efficiency wasrecorded by the G3 while, the control group had the intermediate value. Finally it can be recommended that the supplementation of optigen at level 75gm/ day (1.35% on DM basis) in the diets of Holistien calves.