We have recently used domain-specific monoclonal antibodies (mAbs) to immunofluorescently localize kinesin to vesicle-like structures in the cytoplasm of sea urchin coelomocytes. In order to characterize further these localization patterns we have examined the distribution of kinesin with respect to the arrangement of microtubules (MTs) and various organelles. In double-label experiments involving the immunofluorescent staining of kinesin (using a mixture of the mAbs SUK2, 4 and 5), MTs were labeled with an antiserum against sea urchin tubulin, the endoplasmic reticulum (ER) was labeled with an antiserum against a luminal calsequestrin-like protein, the Golgi apparatus was labeled with rhodamine-wheat germ agglutinin (WGA) or NBD-ceramide, mitochondria were labeled with rhodamine 123, endosomes were labeled with Texas Red-ovalbumin, and lysosomes were labeled with Lucifer yellow or acridine orange. Kinesin-labeled vesicle-like structures were found in the same regions of the cells as MTs and the ER, being widely distributed in motile cells, but restricted to the perinuclear regions of stationary cells. There also appeared to be a correlation between the distribution of endosomes and kinesin staining in a subpopulation of cells. The kinesin binding structures were found occasionally to align in linear arrays, consistent with the idea that kinesin may transport ER and endosomes along linear MT tracks. No clear correlations were observed between the kinesin staining and the distribution of mitochondria, the Golgi apparatus or lysosomes, suggesting that kinesin may specifically associate with only a subclass of organelles in coelomocytes.