DNA topoisomerase I and the isoforms alpha and beta of DNA topoisomerase II were analyzed in different animal cells using a panel of monoclonal antibodies (MoAbs). The beta isoform is a most unstable enzyme. We investigated conditions to stabilize beta isoform because its variability changes according to the derivation of cells. We describe two MoAbs specific to DNA topoisomerase I: the first one recognizes the enzyme in all the species tested including fish; the second one, in contrast, recognizes an epitope present only in mammalian cells. We also found that eight of eight MoAbs against DNA topoisomerase II alpha and five of six against the beta isoform recognize the respective enzymes in all the species tested excluding fish. In addition, MoAbs to the alpha isoform are specific to epitopes not present in the carboxyl third of the enzyme.