Antisera prepared against a 210,000 mol wt microtubule-associated protein (210k MAP) isolated from the human cell line, HeLa, were used to survey a variety of cells and tissues for the presence of immunologically related proteins. The antisera were employed to test extracts of the cells and tissues, using a sensitive indirect immunofluorescence technique applied to polyacrylamide gels. Cross-reactive material of 210,000 mol wt was found in 10 kinds of cells and tissues derived from humans and four lines of cells from monkeys. Indirect immunofluorescent staining was also carried out on fixed cells and showed that the cross-reactive material was localized to interphase and mitotic microtubules as assayed in nine human and seven monkey cell lines. No protein that cross-reacted with 210k MAP antisera was detected in cells and tissues derived from two rodents, an ungulate, a marsupial, or a chicken. Therefore, the 210k MAP isolated from HeLa cells is present in a wide variety of cells and tissues of humans and other primates but is antigenically distinct from MAPs present in lower organisms.