Peroxidase activity was demonstrated in a number of tissues of Atlantic salmon smolts. Using a horseradish peroxidase based heterogeneous, non-competitive, sandwich ELISA designed to detect A. salmonicida, significant background signals were detected in tissues, particularly kidney, taken from disease-free fish. A number of heat treatments were investigated for their ability to reduce this background interference. Pre-treatment at 56degreesC for 10 and 20 min and at 76degreesC for 5 min all reduced the matrix background peroxiclase activity. The treatments at 56degreesC were, however, shown to adversely affect the percentage recovery of spiked A. salmonicida cells. A pretreatment of 76degreesC for 5 min followed by rapid cooling was demonstrated to result in the greatest reduction in matrix background and in the least reduction in percentage recovery.