Small-angle neutron scattering has been used to examine taxol-stabilized microtubules and other tubulin samples in both H2O and D2O buffers. Measurements were made at pH/pD values between 6.0 and 7.8, and observed scattered intensities, I(Q), have been interpreted in terms of multicomponent models of microtubules and related tubulin polymers. A semiquantitative curve fitting procedure has been used to estimate the relative amounts of the supramolecular components of the samples. At both pH and pD 7.0 and above, the tubulin polymers are seen to be predominantly microtubules. Although in H2O buffer the polymer distribution is little changed as the pH varies, when pD is lowered the samples appear to contain an appreciable amount of sheetlike structures and the average microtubule protofilament number increases from ca. 12.5 at pD greater than or equal to approximate to7.0 to ca. 14 at pD approximate to6.0. Such structural change indicates that analysis of microtubule solutions based on H2O/D2O contrast variation must be performed with caution, especially at lower pH/pD.