Feeding lower complexity starter diets has been shown to have no effect on overall growth performance, but may affect immune response and growth during a disease challenge. This study tested the effect of adding fish or corn oil to low or high complexity diets on immune function. In early phases, the low protein diet had lower ADG, ADFI, and Gain:Feed. Fish oil had treatments showed no difference in ADG, tended to  have lower ADFI, and had a significantly better Gain:Feed. The increased Gain:Feed with fish oil may suggest some improvement in immune response.