Ciliates are very good models for studying post-translationally generated tubulin heterogeneity because they exhibit highly differentiated microtubular networks in combination with reduced genetic diversity. We have approached the analysis of tubulin heterogeneity in Paramecium through extensive isolation and characterization of monoclonal antibodies using various antigens and several immunization protocols. Eight monoclonal antibodies and 10 hybridoma supernatants were characterized by: i) immunoblotting on ciliate and pig brain tubulins as well as on peptide maps of Paramecium axonemal tubulin; ii) immunoblotting on ciliate tubulin fusion peptides generated in E coli, a procedure which allows in principle to discriminate antibodies that are directed against tubulin sequence (reactive on fusion peptides) from those directed against a post-translational epitope (non-reactive); and iii) immunofluorescence on Paramecium, 3T3 and PtK2 cells. Twelve antibodies labeled all microtubules in Paramecium cells and were found to be directed against tubulin primary sequences (nine of them being located in the alpha N-terminal domain, one in the beta C-terminal one, and two in alpha and beta central stretches). The remaining ones decorated only a specific subset of microtubules within the cell and were presumably directed against post-translational modifications. Among these, three antibodies are directed against an N-terminal acetylated epitope of alpha-tubulin whereas the epitopes of three other ones (TAP 952 degrees, AXO 58 and AXO 49 degrees) apparently correspond to still unidentified post-translational modifications, located in the C-terminal domain of both alpha- and beta-tubulins. The AXO 49 degrees specificity is similar to that of a previously described polyclonal serum raised against Paramecium axonemal tubulin . The results are discussed in terms of identification and accessibility of the epitopes and immunogenicity of ciliate tubulin with reference to mammalian and ciliate tubulin sequences.