The effect of dietary additions of Echinacea purpurea on the rate of rate of growth, viremia, and ontogeny of the humoral antibody response against porcine reproductive and respiratory syndrome virus (PRRSV) infection was evaluated in weaned pigs. In three replicates, weaned pigs (18 ± 1 day of age) from a PRRSV-naïve herd were randomly allotted to one of four pens (diets) in two rooms, each pen containing five pigs. Each pen of pigs (pens) began one of four dietary treatments 1 week before inoculation with PRRSV: 1) basal ration plus carbadox (0.055 g/kg); 2) basal ration plus Echinacea I (2% of the total ration); 3) basal ration plus Echinacea II (4% of the total ration); and 4) basal ration composed of corn, soybean meal, whey, and supplemented essential vitamins and minerals. E. purpurea was purchased in powder form and determined by chemical analysis to contain 1.35% cichoric acid. Seven days after starting the diets (day 7), all pigs in one room were intranasally inoculated with PRRSV isolate ATCC VR-2332 at a concentration of 10 4 TCID50 /ml. To monitor the effects of diet and PRRSV infection, body weight and blood samples were collected from all pigs at 7-day intervals (day 0 to 42). Serum samples were analyzed for the presence of PRRSV and PRRSV-specific antibodies. All inoculated pigs become infected with PRRSV and all uninoculated pigs remained free of infection. PRRSV-infected pigs had a lower percentage increase in body weight between day 7 and 42 compared with uninfected animals (P<0.06). There were no differences in body weight, average daily gain (ADG), average daily feed intake (ADFI), or gain:feed ratio (G:F) in PRRSV-infected compared with uninfected animals. Animals receiving diets supplemented with Echinacea (treatments 2 and 3), no differences were observed in percentage increase in body weight, ADG, ADFI, and G:F ratio in either the PRRSV-infected and the uninfected pigs. Among PRRSV-infected animals, dietary Echinacea did not affect the rate or level of the ELISAdetectable antibody response day 7 to 42 or the level and duration of PRRSV in serum. Under the conditions of this study, dietary Echinacea did not reverse the growthinhibiting effects of PRRSV, did not exhibit antiviral effects and did not show any evidence of immunostimulatory properties.