Objective: Determine the productive and immune response of Blackbelly lambs infected with gastrointestinal nematodes (GIN) and fed with saccharin. Design/ methodology/ approach: A total of 18 Blackbelly lambs, with an initial live weight (LW) of 13.9  3.2 kg, were randomly assigned to three different treatments (T): T1, anthelmintic treatment  basal diet (CTah); T2, basal diet without anthelmintic treatment (STah); and T3, grazing lambs without anthelmintic treatment (STPS). This experiment followed a completely randomized design with repeated measures over time; mean values were compared using Lsmeans. The parameters evaluated included live weight (LW), fecal egg count per gram (FEC), packed-cell volume (PCV), plasma protein (PP), white blood cell differential count (LEU), and IgA concentration by ELISA with Haemonchus contortus and Trichostrongylus colubriformis antigens.Results: STah and CTah lambs showed higher FEC (885  142) and LW (29.73  5.06 kg). Grazing lambs (STPS) had lower PCV (26.4  0.5%) compared to the STah and CTah lambs (27.4 to 28.4%) due to the high prevalence of H. contortus. The IgA concentration in grazing lambs ranged from 20.2 to 24.5% of the positive standard serum titer. The feedlot lambs (STah and CTah) showed values close to 5%. Study limitations/implications: Due to anthelmintic resistance problems, it was impossible to maintain grazing lambs free of infection; therefore, this group was not included. Findings/conclusions: Saccharin increases sheep resilience and achieves adequate weight gains in parasitized lambs