The objectives of this study were to determine the effects of adding a mixture of malate and yeast culture (Saccharomyces cerevisiae) on cellulolytic, amylolytic, proteolytic and ureolytic activity in the rumen and on the growth performance of lambs. Eight native breed (Lory-Bakhtiary) lambs aged approximately 7 months (four males and four females) were divided into two groups by sex. They were kept in individual pens and fed a total mixed ration (TMR) based on lucerne hay-wheat straw and concentrate. The two groups of lambs were randomly assigned to each of the dietary treatments in a cross-over design: control (without supplementation) and supplemented (with malate and yeast included in the TMR at a level of 10 g ·kg^-1). The experiment was divided into 2 periods, in which the lambs received one diet in the first three-week period, after which they were transferred to the other diet for another three weeks. Urease, cellulase, protease and amylase activities were determined in rumen fluid and rumen microbial biomass. Rumen samples were taken 3-4 h after feeding at the end of each period. The weights of lambs were recorded at the beginning and end of each period. RNA equivalents (RNA-e) in rumen fluid and urine were measured for determination of microbial status in different samples. Supplementation with the malate-yeast mixture increased daily weight gain (from 196 in controls to 259 g/d) but did not affect (P>0.05) feed intake (0.970 vs 0.929 kg DM/d for the control vs the supplemented lambs, respectively). Higher cellulase activity was observed in lambs on the malate-yeast diet compared with the control lamb diet. A significant but moderately positive correlation was found between cellulase and protease activities (r=0.5188; P=0.0395). No significant differences were observed between the two treatments for dry matter, neutral detergent fibre and crude protein apparent digestibilities (P>0.05). Neither was microbial biomass (based on RNA-e concentrations) affected by the malate-yeast treatments (P>0.05).