Ladderlectin is a putative defense protein originally isolated from rainbow trout serum. This thesis describes the bacterial-binding properties of rainbow trout ladderlectin (RT-LL), examines individual variability of RT-LL concentrations in healthy trout and temporal alterations of RT-LL concentration during the acute phase response (APR). Binding assays with intact bacteria demonstrated that RT-LL binds a range of relevant fish pathogens. Development of an indirect ELISA to measure plasma RT-LL concentrations revealed significant (' P' < 0.0001) variability between individual trout in two separate populations. During the APR, RT-LL concentrations did not increase in fish either injected with bacterial products or experimentally infected with ' Aeromonas salmonicida.' In contrast, a modest, but statistically significant ('P' < 0.05), reduction of RT-LL concentration was observed in experimentally infected fish. These studies demonstrate that RT-LL is variably expressed in plasma of rainbow trout and may decrease during clinical infection, but is not induced during the APR.