This study was performed to investigate effects of low fat diet and saturated fat supplementation on the function of the immune system. Forty male BALB/c mice average-weighing 15g were divided into two dietary groups: 0.7% safflower oil group and 4.3% beef tallow & 0.7% safflower oil group. Results are as follows; 1) Food intake, body weight, organ weight, agglutination test, differential white cell count and histological examination of spleen were not different in two dietary groups during the experimental period. 2) Delayed-type hypersensitive test of the mice fed 4.3% beef tallow & 0.7% safflower oil was significantly higher than that of the mice fed 0.7% safflower oil ($alpha$=0.05). 3) Plaque forming cell was significantly reduced at 10th week compared to 7th week in both groups($alpha$=0.05). Although there was no significant difference between two groups. 0.7% safflower oil groups showed slightly higher plaque forming cell than 4.3% beef tallow & 0.7% safflower oil group.