In previous studies in Drosophila, Nielsen et al. hypothesized that the beta tubulin C-terminal axonemal motif ;EGEFXXX', where X is an acidic amino acid, is required for ciliary function and assembly (Nielsen et al., 2001, Curr. Biol. 11, 529-533). This motif is present in some but not all mammalian beta tubulin isotypes. We therefore investigated whether this motif is important in ciliary function in mammals. In a preparation of isolated, ATP-reactivated bovine tracheal cilia, we found that monoclonal antibodies directed against the C-terminus of betaI, betaIV and betaV tubulin blocked ciliary beating in a concentration dependent manner. Antibodies against other epitopes of beta tubulin were ineffective, as were antibodies against alpha tubulin. Peptides consisting of the axonemal motif and motif-like sequences of these isotypes blocked ciliary beating. These results suggest that the axonemal motif sequences of betaI, betaIV and betaV tubulin are essential for ciliary function. Peptides consisting of corresponding C-terminal sequences in alpha tubulin isotypes were also ineffective in blocking ciliary beating, which suggests that the C-terminus of alpha tubulin is not directly involved in cilia function in mammals.