The cytotoxic activity of natural killer (NK) cells isolated from peripheral blood of 20 healthy donors and 34 patients with multiple sclerosis (MS) against labelled with H3-uridine target cells K-562 before and after their 1 hr treatment with reaferon (RF), T-activin (TA), myelopid (MP), opioid preparation dalargin (DL) as well as with combinations of TA, MP and DL with RF was studied in 14 hrs cytotoxic test. It has been shown that combination of RF with TA, MP and DL changed the regulatory action of these peptides on NK cell activity in healthy donors in vitro. The same combination of the preparations in patients with MS caused another changes in regulation of NK activity by them because NK cells in MS patients had had initially changed sensitivity to action of these regulatory polypeptides.