The physiology bioactive factor, FL5, was obtained from salted cooking water of fish larvae. BALB/c mice were then fed with pellets that contained 0.25, 0.5, and 1% (w/w) dosages of FL5. The feeding was ad lib for 10 weeks. The results showed that the pellets with the FL5 did not affect the body weight, fed efficiency and immunoglobulin M (IgM) concentration of mice. No significant differences in IgM levels were observed between the FL5 group and the controlled group. Mice fed with the 1% group had increased proliferation of spleen cells. Results from this study suggest that the FL5 may regulate the immune system in BALB/c mice.