Vaccines are commonly used in salmonid aquaculture as a method of disease prevention. Although there is a substantial amount of published research regarding the immunological and physiological effects following the injection of different polyvalent vaccines and DNA vaccines, there are no published reports examining the physiological and immunological effects of concurrent vaccine injection, which is the situation encountered in aquaculture. Using key immunological parameters such as lysozyme activity and specific antibody titres we examined the short-term activation of the immune response of cultured Atlantic salmon (Salmo salar L.) following concurrent injection with a traditional, polyvalent, oil-adjuvanted vaccine (AV) and an IHNV-specific DNA vaccine (DV). Our results indicate that different aspects of the innate and adaptive immune responses are influenced in either a positive or negative manner. While concurrent vaccine injection elicited an increase in lysozyme activity, changes in antibody titre (Ab) were antigen specific. The production of anti-Aeromonas salmonicida Abs was significantly greater in the combined vaccine group at 296 degree days post-vaccine injection (dd pvi), while the production of anti-Listonella anguillarum Abs was significantly greater at 106 dd pvi in the combined vaccine group. Of even greater interest was the apparent delay in production of IHNV-specific neutralizing antibodies (NAb) when the DV was injected concurrently with the polyvalent AV. The results indicated that concurrent injection of a polyvalent oil-AV and a DV can be beneficial to the production of antibodies however, the specific anti-viral response may be delayed. Fish & Shellfish Immunology 28 (2010) 579-586. Lisa A. Skinner , S.E. LaPatra , A. Adams , K.D. Thompson , S.K. Balfry , R.S. McKinley , P.M. Schulte.( Faculty of Land and Food Systems, University of British Columbia, 2357 Main Mall, Vancouver, BC, Canada V6T 1Z4 Clear Springs Foods Inc., 1585A Clear Lakes Grade, Buhl, ID 83316, USA Institute of Aquaculture, University of Stirling, Stirling, Scotland, UK FK9 4LA DFO/UBC Centre for Aquaculture and Environmental Research, 4160 Marine Drive, West Vancouver, BC, Canada V7V 1N6 Department of Zoology, University of British Columbia, 6270 University Blvd, Vancouver, BC, Canada V6T 1Z4).