The paper devoted to the 120th anniversary of A.A. Bogomoletz birth deals with the developmental stages of the cytotoxin (xenogenic antibody) doctrine from creating the anti-reticular cytotoxic serum (ACS) of Bogomoletz to the use of monoclonal .. I. Aeaen..aa, .. .. .i..e 10 ISSN 0201-8489 O.c.ie. .o.i., 2001, O. 47, . 2 antibodies both in biological investigations and in medicine. The use of ACS for treating various diseases and the mechanisms of its effect; obtaining other anti-organ and anti-tissue cytotoxic sera and establishing their experimental effects; the use of a set of cytotoxic sera, taken in both inhibitory and stimulating doses, in medicine and agriculture, as well as the drawbacks of cytotoxic sere (polyclonal xenogenic antibodies) are described. The experience of using monoclonal antibodies to identify the structures of protein antigenic determinants and to establish their role in different diseases are analysed. The use of monoclonal antibodies for treating various diseases is reviewed. The advances in the techniques of obtaining monoclonal antibodies are noted.