Eight lactating Holstein‐Friesian dairy cattle were randomly allotted as pairs to either a control or a treatment group fed technical pentachlorophenol (penta) from 6 ±1 wk post‐partum for about 135 d (0.2 mg/kg‐d for 75–84 d followed by 2.0 mg/kg‐d for 56–62 d). Jugular blood was drawn periodically for immunologic studies. Leukocyte differentials and lymphocyte subpopulations (e.g., T and B cells) were enumerated for each blood specimen. Several in vitro and in vivo assays were conducted to evaluate lymphocyte functions, including (1) quantitation of serum immunoglobulins G, M, and A; (2) induction of blastogenesis in vitro, using concanavalin A and leukoagglutinin as mitogens; (3) measurement of extent of skin reaction to injected purified protein derivative in BCG‐sensitized cattle to evaluate delayed hypersensitivity; and (4) quantitation of antibody formation in response to injected sheep red blood cells. Neutrophil function was evaluated by latex particle phagocytosis and chemiluminescence. The results showed no statistically significant differences between control and penta‐treated cattle during either treatment period. Also, no histopathologic changes were noted in lymphoid tissues including spleen, thymus, and lymph node.