Echinacea is one of the most widely used over-the-counter herbal preparations that purport to "improve immune system function", especially when taken as a short course of therapy (6-8 weeks). Since many purchasers are older individuals, a double-blind, placebo-controlled study was performed to investigate whether Echinacea could affect total and differential white cell counts, phagocytic activity and interleukin (IL-2) levels in 12-month-old, healthy, male Sprague-Dawley rats when administered over an 8-week period. Echinacea (50 mg/kg of aerial parts) mixed with peanut butter or peanut butter alone was fed to 16 rats, which were receiving regular food and water ad libitum. Cell counts and immune functions were determined on rat tail vein blood on a weekly basis. Echinacea significantly increased circulating total white cell counts during the first 2 weeks of administration, and IL-2 levels during the final 5 weeks of the study period (p<0.05). Differential counts were altered during the entire 8-week study, with mononuclear cells significantly increased to the detriment of granulocytes (p<0.05). No such changes were observed in animals given peanut butter alone. No difference was observed in phagocytic function between animals given Echinacea or peanut butter alone. These studies suggest that aerial components of Echinacea affect both mononuclear cell levels and circulating IL-2 levels in older animals.