CasuL is a lectin (carbohydrate-binding protein) isolated from the leaf pinnulae of Calliandra surinamensis that is toxic against cancer cells. In this study, the effects of CasuL on the activation of immune cells were evaluated in BALB/c mice splenocytes. Assays measuring the changes in cytosolic calcium concentration ([Ca²⁺]_cyt), mitochondrial membrane potential (ΔΨm), and reactive oxygen species (ROS) levels associated with cell viability, proliferation, and cytokine and nitric oxide production were performed. The lectin (3.12–100 μg/mL) did not induce apoptosis or necrosis of splenocytes, and treatment for 48 h at 12.5 μg/mL stimulated cell proliferation. High cytosolic ROS levels were found in cells incubated with CasuL (12.5 μg/mL), but it did not affect [Ca²⁺]_cyt, mitochondrial ROS, and ΔΨm levels. Furthermore, CasuL promoted high IL-2 and TNF-α production but did not affect nitric oxide release. In conclusion, CasuL was able to promote oxidative stress in mouse immune cells without inducing cell damage, and stimulated proliferation and cytokine production. These findings suggest the potential use of CasuL in future antitumoral and immunological targets.