A study was made of the cytotoxic activity of natural killers (NK) against 3H-uridine labeled K-562 cells and its variability. 56 patients suffering from multiple sclerosis (MS), 33 healthy donors of different age and 34 patients with remitting MS and MS of different standing were entered into the study. The most remarkable lowering of the NK activity was seen in patients aged 41 to 50 years. In patients aged 51 to 59 years, the NK activity was not different from that in a group of healthy donors. The most pronounced increase of the variability index (VI) was recorded in patients aged 41 to 50 years. In the senior age group (from 51 to 59 years), the VI of the NK activity was 1.7 to 2.8 times as reduced as compared to the healthy control. It has been revealed that the changes in the NK activity and its variability in the respective patients' groups are of concomitant nature and that the estimates undergo fluctuations in the counterphase. During studies of the NK activity as dependent on the disease standing, NK deficiency was found to be associated with the gravity of the patient's clinical status and long disease standing. A possible relation of the changes in the NK activity and its variability to the disease pathogenesis is under discussion as are the compensation mechanisms that get triggered during depletion of the system according to the secondary deficiency type.