The primary focus of the current study was to determine the potential benefits of supplementing sheep diet with white button mushrooms (WBM) in terms of growth, health and the kinetics of systemic CD4+ CD8+ memory T lymphocytes in lambs. Forty-five female lambs (Lika breed) were divided into three groups: A – the control group fed on a free-range pasture for the 222 days of the experiment, while groups B and C were housed in a separate facility for 42 days and fed either a commercial feed mixture (FM) or a FM supplemented with 15% of freshly prepared WBM, respectively, and ad libitum forage. For the remaining 180 days of the experiment, both groups (B and C) of lambs were kept free-range and fed pasture only. The lambs were monitored daily starting on Day 0 (or 90 days of age) before the treatments, weighed and blood sampled on Days 0, 21, 42 and 222, and were clinically observed for the incidence/severity of diarrhea and/or other signs of disease. In addition to morbidity, mortality was also monitored, and dead lambs were examined for gross pathology changes. The lambs fed FM supplemented with WBM (group C) had significantly higher body weight gain (P<0.05) on Days 42 and 222. They were neither diarrheic nor had any mortality cases throughout the experiment. Also, these lambs had a significantly increased (P<0.05) proportion of CD4+CD8+ T cells on Days 42 and 222. The data obtained supported our assumption of the efficacy of dietary WBM in the immunostimulation of CD4+CD8+ memory T lymphocytes in lambs, resulting in protection against on-farm diarrhea and providing an increased growth rate.