en fish disease fish disease پژوهشي Orginal research papers This study was conducted to evaluate the effects of different levels of a commercial herbal oil mixture (<em>Mix-oil^&reg;</em>) on the hematological, biochemical and immunological responses of rainbow trout as well as the resistance against infection with <em>Yersinia ruckeri</em>. The <em>Mix-oil^&reg;</em>includes, <em>Thymus vulgaris</em>, <em>Origanum vulgare</em> and <em>Eucalyptus </em>sp. essential oils. Experimental diets were supplemented with <em>Mix-oil^&reg;</em> at 0 (control group), 50, 200 and 400 ppm. Fish with an initial weight of 31.0&plusmn; 0.1 g were divided into four groups and reared for 56 days. The anti-bactericidal activity of <em>Mix-oil^&reg;</em>revealed that the minimum inhibitory concentration (MIC) value for <em>Streptococcus iniae</em>, <em>Aeromonas hydrophila</em> and <em>Lactococcus garviae</em> was 6.25 &mu;l ml^-1 while it was 12.5 &mu;l ml^-1 for <em>Y. ruckeri</em>. Significant improvements in red blood cell count, hematocrit, immunoglobulin, lysozyme, total protein, complement component C3 and C4, and alternative complement pathway (ACH₅₀) in fish fed with 400 ppm <em>Mix-oil^&reg;</em> were observed compared with the control group (<em>p</em><0.05). No significant difference was observed between different treatments in terms of MCV, MCH, MCHC levels and hemoglobin percentage aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels (<em>p</em>>0.05). The results suggested that dietary administration of <em>Mix-oil^&reg;</em> at the level of 400 ppm could enhance hematological parameters and immune function of rainbow trout as well as the resistance against <em>Y. rukeri</em>. Thus, using <a name="_Hlk515727322"><em>Mix-oil^&reg;</em> </a>as an immunostimulant is recommended for farmed rainbow trout. Herbal oil mixture, (Mix-oil®), Non-specific immune responses, Rainbow trout, Yersinia ruckeri. 1304 1318 http://jifro.ir/browse.php?a_code=A-10-2181-3&slc_lang=en&sid=1 A. Bababaalian Amiri alirezababaalian@gmail.com 100319475328460034059 100319475328460034059 No Department of Aquatic Animal Health and Diseases, Science and Research Branch, Islamic Azad University, Tehran, Iran G. Azari Takami Takami85@hotmail.com 100319475328460034060 100319475328460034060 Yes Department of Aquatic Animal Health and Diseases, Science and Research Branch, Islamic Azad University, Tehran, Iran M. Afsharnasab mafsharnasab@yahoo.com 100319475328460034061 100319475328460034061 No Department of Aquatic Animal Health and Diseases, Iranian Fisheries Science Research Institute (IFSRI), Agricultural Research Education and Extension Organization (AREEO), Tehran, I.R. Iran A. Zargar azargar@ut.ac.ir 100319475328460034062 100319475328460034062 No Department of Aquatic Animal Health and Diseases, University of Tehran, Tehran, Iran