AbstractIn this study, we report differences in the observed gliding speed of microtubules dependent on the choice of bovine casein used as a surface passivator. We observed differences in both speed and support of microtubules in each of the assays. Whole casein, comprised of [alpha]~s1~, [alpha]~s2~, [beta], and [kappa] casein, supported motility and averaged speeds of 966 ± 7 nm/s. Alpha casein can be purchased as a combination of s1 and s2 and supported gliding motility and average speeds of 949 ± 4 nm/s. Beta casein did not support motility very well and averaged speeds of 870 ± 30 nm/s. Kappa casein supported motility very poorly and we were unable to obtain an average speed. Finally, we observed that mixing alpha, beta, and kappa casein with the proportions found in bovine whole casein supported motility and averaged speeds of 966 ± 7 nm/s.