SUMMARYThe effect of antibiotics on lymphocyte HL‐A antigensin vitrois of variable character. All antibiotics under examination suppressed the absorption capacity of HL‐A antigens after 2 hr of lymphocyte treatment at 37°C. Ceporin and Kanamytrex inhibited even the cytotoxic reactivity of H1‐A antigens after 15–30 min of lymphocyte treatment. Chloramphenicol, aureomycin, streptomycin and oleandomycin, on the contrary, increased the specific cytotoxic reactivity of HL‐A antigens after 15–30 min, after 1 hr they were ineffective for HL‐A antigens, and after two or more hours they produced polyreactivity. Penicillin and erythromycin produced polyreactivity after only 15–30 min. The results show that for the follow‐up of the drug effect on HL‐A antigens the absorption test rather than the cytotoxicity test is of importance. The suppressed absorption capacity of HL‐A antigens caused by the action of antibiotics proves their inactivation effect on the lymphocytes. The possibility of an analogous effect of antibiotics on lymphocyte HL‐A antigens, even after administration t