KovafiU H., F. KovafiU, A. Fi‰erova, P. Zelniakova, E. Matalova: Biochemical Markers of Lymphocyte Maturation. Acta Vet Brno 2002, 71: 503-508. We studied maturation-associated changes of cell membrane Na + , K + - and Mg 2+ -ATPase activities and gamma-glutamyltranspeptidase activity (GGT) during prenatal and postnatal development of lymphocytes in pig thymus and lymph nodes, i.e. primary or secondary lymphoid organs. Furthermore, we analyzed the developmental changes in expression of alpha subunits (Gs, Gq/11) of membrane heterotrimeric GTP-binding proteins. We demonstrated marked developmental decrease of both Na + , K + - and Mg 2+ -ATPase activities in thymus and lymph node lymphocytes. On the other hand, we found elevated membrane gammaglutamyltranspeptidase (GGT) activity as a function of age in lymph node lymphocytes in contrast to declined GGT in thymus. These findings are probably associated with antibody formation related to antigenic pressure during postnatal development. Finally, we assayed developmental changes of G alpha (s) and G alpha q/11 subunits in thymus and lymph nodes. There were ontogenetic specifities in expression of G alpha (s) or G alpha q/11 and appropriate effector enzyme systems generating of 2 nd messengers cAMP or 1,4,5 IP 3 , respectively, in cell signal transduction. The results of this study contribute to our understanding of the dynamic changes of both enzymes and G alpha subunit profiles at the level lymphocyte cell surface linked to maturation events in ontogenetic development. Pig, ontogeny, thymus, lymph nodes, lymphocytes, ATPases, gamma-glutamyltranspeptidase, heterotrimeric GTP-binding protein, G-protein It is generally accepted that T and B lymphocytes and their subpopulations are characterized by morphological and phenotypic markers, including surface antigens or receptors. Their cell surface enzyme equipment can be different as well. These enzymes belong to various groups, e.g. phosphatases, nucleotidases, kinases, peptidases and glycosidases with possible functions in cell activation, maturation, differentiation, molecules transducing cell signal and adhesive structures ( Naquet and P ierres 1991; Benrezzak et al. 1999).