Variation in α-cysteine proteinase inhibitor levels in human sera were investigated with special attention to the effect of pregnancy and diseases. The inhibitor level in 111 pregnant women, examined by our previous method, increased as the pregnancy advanced. The correlation coefficient between the level and the duration, r, was 0.578 with the level of significance p < 0.001. The mean value for 50 pregnant women in the third trimester of gestation was 0.49 ± 0.04 unit/mL, which was 47% higher than the control value, 0.33 ± 0.04 unit/mL ( p < 0.001). However, the levels in patients with myoma of uterus, endometritis, cervical cancer, ovarian cyst and ovarian cancer were not significantly different from the control value. From these results, it was concluded that pregnancy enhanced appreciably the α-cysteine proteinase inhibitor level in serum.