NuMA is a protein involved in maintenance of nuclear structure and in the assembly of the mitotic spindle. Expression of amino-terminal deletion mutants results in a phenotype identical to that caused by a temperature-sensitive defect of RCC1 (regulator of chromosome condensation). Here we describe the isolation of NuMA protein from HeLa cells under mild conditions as a prerequisite to study its interactions with elements of the RCC1-Ran regulatory pathway. In an overlay assay, NuMA did not bind Ran.[gamma-32P]GTP. Thus it is clearly different from Ran.GTP binding proteins of similar M(r).