Objective: Determine the productive and immune response of Blackbelly lambs infectedwith 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 celldifferential count (LEU), and IgA concentration by ELISA with Haemonchus contortusand Trichostrongylus colubriformis antigens.Results: STah and CTah lambs showed higher FEC (885 ± 142) and LW (29.73 ± 5.06kg). Grazing lambs (STPS) had lower PCV (26.4 ± 0.5%) compared to the STah andCTah lambs (27.4 to 28.4%) due to the high prevalence of H. contortus. The IgAconcentration in grazing lambs ranged from 20.2 to 24.5% of the positive standardserum titer. The feedlot lambs (STah and CTah) showed values close to 5%.Study limitations/ implications: Due to anthelmintic resistance problems, it wasimpossible to maintain grazing lambs free of infection; therefore, this group was notincluded.Findings/ conclusions: Saccharin increases sheep resilience and achieves adequateweight gains in parasitized lambs.