The identity of two microtubule-associated proteins, MAP3 and MAP4, was verified both immunologically and biochemically. MAP3 was enriched from the heat-stable fraction of rat brain extracts by reverse-phase HPLC and preparative two-dimensional gel electrophoresis, Both MAP3 and MAP4 antibodies reacted with the corresponding spots on two-dimensional Western blots. Amino acid sequences of internal peptides derived from rat MAP3 matched with corresponding sequence stretches of mouse MAP4. In the kidney cortex, the MAP3 antibody stained not only glomerular podocytes but also interstitial cells. This distribution pattern of MAP3 is identical to that of MAP4 reported previously. These results indicate that MAP3 and MAP4 are identical.