Plasma samples obtained by continuous withdrawal for a 10-hour period in normal menstrual cycle were assayed for human luteinizing hormone (hLH) by both the in vitro bioassay and improved radioimmunoassay (RIA). The pulsatile patterns of both biological and immunological hLH levels were demonstrated with high frequency and low amplitude in the follicular phase and with low frequency and high amplitude in the luteal phase in the present study, as seen in the literature, although the assay procedures were different. The basal levels of immunoreactive hLH were 17.99mIU/ml with 95% confidence limits at 14.49 and 22.34mIU/ ml in the follicular phase and 10.79 (6.62 : 17.58) mIU/ml in the luteal phase. These two values were comparable to those in the literature. On the other hand, those of bioreactive hLH in the corresponding phases were 21.68 (15.21 : 30.90) mIU/ml and 6.22 (4.94 : 7.83) mIU/ml, respectively. The differences of the basal levels in these two phases were more pronouncedly assessed by the bioassay than by the RIA. The ratios of biological to immunological hLH activities (B/I ratios) at the trough (base) were over the unity (1.21 (1.03 : 1.41) ) in the follicular phase, whereas those in the luteal phase were decreased down to 0.58 (0.44 : 0.76). This depression is likely due to the poor specificity of the antiserum employed in the RIA. The half disappearance times (t1/2's) of the immunoreactive hLH were shortened significantly in the preovulatory phase(83.7(64.3 : 103.1)min)in comparison with those of the bioreactive hLH(122.9(98.2 : 147.6) min). The B/I ratios at the peaks of the pulses were depressed down in the preovulatory phase. The possible existence of hLH with immunoactivity and no, or little, bioactivity was suggested from these two evidences in this stage of menstrual cycle. The difference in the t1/2's of bioreactive hLH in the follicular and luteal phase (103.6 (94.7 : 112.5) min and 87.2 (72.5 : 101.9) min, respectively) indicated proportional differences of hLH subpopulations with various clearance rates in these two phases.