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Suppression of growth during infection may aid resource allocation towards effective immune function. Past work supporting this hypothesis in salmonid fish revealed an immune-responsive regulation of the insulin-like growth factor (IGF) system – an endocrine pathway downstream of growth hormone (GH). Skeletal muscle is the main target for growth and energetic storage in fish, yet little is known about how its growth is regulated during an immune response. We addressed this knowledge gap by characterising muscle immune responses in size-matched coho salmon (Oncorhynchus kisutch) achieving different growth rates. We compared a wild-type strain with two GH transgenic groups from the same genetic background achieving either maximal or suppressed growth – a design separating GH's direct effects from its influence on growth rate and nutritional state. Fish were sampled 30 h post-injection with …