chromokinesin is a developmentally down-regulated gene with specific expression in proliferating cells during embryonic chick development. It encodes a DNA-binding motor protein localized along the chromosome arm during mitosis, suggesting that the protein may be a component of the long-observed, yet poorly understood 'ejection force' hypothesized to be involved in controlling the direction and speed of chromosome movement. We have isolated human chromokinesin; with affinity-purified antibodies we demonstrated immunocytochemically that Chromokinesin was present at a much higher level in cultured retinoblastoma cells than in primary cultures of human dermal fibroblasts. The increase in immunoreactivity was particularly prominent in interphase cells, whereas in primary cultures of fibroblasts immunopositive cells were predominantly M-phase cells. These observations imply a deregulation of chromokinesin in retinoblastoma cells. Data presented here may be useful in designing strategies to modulate chromosome movement and cell proliferation with either antisense oligonucleotides or specific antibodies, and hence may set the stage for further investigations of the involvement of chromosome motor molecules in mitosis under normal and pathological conditions.