Copines are a recently identified group of proteins characterized by two Ca(2+)-binding C2-domains at the N terminus and an A-domain at the C terminus. Although pEST sequences indicate the existence of at least seven copines in man, only copines I, III, and VI have been identified at protein level. Here, we describe the isolation of copines I and III in the cytosol of human neutrophils by use of Ca(2+)-induced hydrophobic chromatography. This is the first demonstration that copines are coexpressed in the same cell. We found that copine III exists in the cytosol of human neutrophils as a monomer with a blocked N terminus. Copines I and III undergo conformational changes upon Ca(2+) binding that lead to exposure of hydrophobic patches. Examination of RNA from 68 human tissues demonstrated that copines I-III are ubiquitously expressed whereas copines IV-VII each has a more restricted and individual expression profile. Expression of copines I-III was also demonstrated in neutrophil precursors from bone marrow. Copine I was uniformly expressed at all stages of neutrophil differentiation, whereas copine II and even more so, copine III were expressed in the more immature neutrophil precursors, which indicates an individual function of these copines.