:Twenty seven Barki lambs, seven months old, with live body weight of 33.33±2.21kg were used to investigate the effects of partial replacement of berseem hay by biologically treated rice straw on chemical composition, growth performance, digestibility, rumen fermentation, some blood criteria and immune status of sheep. The animals were divided into three comparable groups. Three rations were prepared with the same roughage: concentrate ratio of 40:60% and iso-nitrogenous (R1- Control (60% CFM + 40% berseem hay, R2- 60%CFM + 40% Roughage (70% berseem hay + 30% rice straw treated with Trichoderma viride), R3- 60%CFM + 40% Roughage (70% berseem hay + 30% rice straw treated with Aspergillus fumigatus). Lambs body weight was recorded at the start of the experiment and thereafter regularly at 14 days interval. After completion of the feeding trial (12 weeks) , four animals per treatment were used in a metabolism trial. Obtained results showed that treated groups showed significant (P< 0.5) improvement in OM, CP, CF and NFE, while DM, EE and Ash did not differ significantly. The body weight non-significantly increased here-after up to week six; then increases started to the reach the significant level. The final body weight was 43.45, 45.5 and 45.35kg; total gain was 10.18, 12.07 and 12.14kg for the respective groups. Average daily gain was 121, 143.67 and 144.44g/d., CP, CF, NFE and EE digestibility were significantly (P< 0.5) increase in treated groups, while DM digestibity did not differ significantly by treatments. All treatments had similar nutritive values, being 63.00, 63.83 and 63.08% for TDN and 10.11, 10.57and 10.39% for DCP, respectively. Values of NB were 4.03, 4.98 and 5.03g/d for the three experimental groups, respectively.  The biological value of the dietary protein was higher for groups AF (35.66%) and TV (35.48%) and lowest for control group (30.00%). Treating rice straw with either TV or AF fungus did not affect the rumen pH values at any time after feeding. At three hours from feeding VFA production increased from 10.12 in C diet to 12.98 for TV and 12.35meq/dl for AF. Rumen NH3 concentration increased from 16.25 in C diet to 18.65 for TV and 18.39mg/dl for AF at three hours from feeding, similar trend was found at six hour post feeding. Generally, all values were within the normal values of blood characteristics of sheep. Serum total protein, albumin and glucose were  significant (P< 0.5) increase in treated groups than control group, while treatments had no significant effect on globulin, urea, creatinine, AST and ALT. Immunological parameters (IgA, IgG and interleukin 2) were within normal ranges and did not differ significantly.