The objective of this study was to compare doses of a polyherbal mixture (Phaseolus mungo with Linum usitatissimum, OptiLysine® from Nuproxa México, Indian Herbs) and from rumen-protected lysine (RPL; AjiPro®-L) on lamb growth, changes in rumen protozoa and blood biochemistry. Fifty-six Pelibuey x East Friesian lambs (initial live weight 21.97 kg ± 4.29) were randomly assigned to treatments consisting of a control group, or three daily doses of lysine sources: polyherbal mixture (5, 10 and 15 g/d) and RPL (5, 10 and 15 g/d) dosed daily orally for 45 days. The lambs were fed individually with a basal ration (13.2% CP, 2.26 Mcal ME/kg) with an estimated duodenal flow in the basal diet of 6.45 g/d of lysine and 2.32 g/d of methionine. The doses of both sources did not affect the productive performance with the exception of the intake that showed a quadratic response with the RPL (p<0.05). The polyherbal mixture linearly stimulated the Entodinios population (p<0.10) and reduced the Holotrichs population (p<0.05), while the RPL linearly reduced the Holotrichs population (p<0.05). In blood biochemistry, the polyherbal mixture only affected lactic dehydrogenase (quadratic effect p<0.05) and RPL increased alkaline phosphatase (linear p<0.05) and decreased globulin (quadratic effect p<0.05). The results indicate that the polyherbal mixture and the rumen protected lysine did not improve the growth of the lambs, but the protozoan populations of the rumen were affected.