With a rabbit antiserum specific for high molecular weight microtubule-associated proteins (HMW/MAPs: MAP1 and MAP2), centrosomes in interphase and mitotic HeLa cells were visualized by indirect immunofluorescence. Pre-immune serum did not stain a similar organelle. Microtubules were observed to originate from the centrosomes, demonstrating their microtubule organizing (MTOC) function. The MTOC antigen(s) to which the anti-HMW/MAP antiserum reacted was resistant to Colcemid, Triton-X-100, Nonidet P-40 and Brij 35. Immunofluorescence localization of the MTOCs was blocked by pre-incubation of the antiserum with a MAP1--MAP2 preparation but not with tubulin, actin, or a 58 K intermediate filament protein.