RECENT evidence suggests that the force for poleward movement of chromosomes during mitosis is generated at or close to the kinetochores1,2. Chromosome movement depends on motion relative to microtubules, but the identities of the motors remain uncertain1,3-7. One candidate for a mitotic motor is dynein, a large multimeric enzyme which can move along microtubules toward their slow growing end8-11. Dyneins were originally found in axonemes of cilia and flagella where they power microtubule sliding. Recently, cytoplasmic dyneins have also been found12, and specific antibodies have been raised against them. The cellular localization of dynein has previously been studied with several antibodies raised against flagellar dynein, but the relevance of these data to the distribution of cytoplasmic dynein is not known13-17. Antibodies raised against cytoplasmic dyneins have shown localization of dynein antigens to the mitotic spindles in Caenorhabditis elegans embryos (Lye et al., personal communication) and punctate cytoplasmic structures in Dictyostelium amoebae18. Using antibodies that recognize subunits of cytoplasmic dyneins, we show here that during mitosis, cytoplasmic dynein antigens concentrate near the kinetochores, centrosomes and spindle fibres of HeLa and PtK1 cells, whereas at interphase they are distributed throughout the cytoplasm. This is consistent with the hypothesis that cytoplasmic dynein is a mitotic motor.