Lysozyme activity is a common measurement of innate immunity. It has also been used to investigate genetic variation and an animal's responses to factors such as stress, infections and variations in diet. This research demonstrates the inherent variation in lysozyme activity in unstimulated coho salmon (Oncorhynchus kisutch). The role of maternal contribution, early life stage development and fish mass are considered. Genetic variation within and between strains of coho was found to be significant at selected life stages. Our results indicate that strain differences in lysozyme activity are more accurately measured by comparing the genetic variation after the eyed stage, when maternal effects are reduced. A positive correlation between plasma/serum lysozyme activity and fish mass is reported here. In summary, this study shows the role of maternal, developmental stage and size in lysozyme activity in fish, and emphasizes the importance of considering such variables when measuring the variability of lysozyme activity in fish.