In neutrophils, the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) induced the translocation of the Ca(++)- and phospholipid-dependent protein kinase, protein kinase C (PK-C) from the soluble to the particulate fraction. At the same time there was a corresponding increase in the amount of Ca(++)- and phospholipid-independent protein kinase activity recovered in the soluble fraction. This soluble Ca(++)- and phospholipid-independent protein kinase presumably reflects proteolytic activation of the particulate associated PK-C. Bone marrow and undifferentiated HL-60 cells also translocated PK-C to the particulate fraction in response to TPA but did not accumulate the soluble Ca(++)- and phospholipid-independent form of the enzyme. Similar results were obtained using HL-60 cells induced to differentiate with dimethyl sulphoxide (DMSO), recombinant human granulocyte-macrophage colony-stimulating factor (rh GM-CSF) or 1 alpha,25-dihydroxyvitamin D3. There was also no significant change in either the number or time of expression of differentiation-specific cell surface antigens observed on HL-60 cells induced to differentiate with either DMSO, 1 alpha,25-dihydroxyvitamin D3 or TPA in the presence of cyclosporin A, an agent reported to inhibit the proteolytic breakdown of PK-C to the Ca(++)- and phospholipid-independent form. Likewise, cyclosporin A did not affect the rate of extent of differentiation of primary bone marrow cell cultures. These results suggest that the proteolytically activated and phospholipid-independent form of PK-C is probably not involved in haemopoietic cell differentiation.