Two dimensional gel electrophoresis of proteins from HL-60 human leukaemia cells treated with bistratene A, a specific activator of protein kinase C (PKC)δ, was performed in conjunction with sequencing in order to identify components of the signal transduction pathway of this isoform of PKC. Stathmin (oncoprotein 18) was identified in this way and the phosphorylation of this protein after treatment with bistratene A, was confirmed by Western blotting of 2D gels. Since stathmin has phosphorylation sites for mitogen activated protein (MAP) kinases, cyclin dependent kinases and calcium/calmodulin dependent protein kinases, it is assumed that one of these enzymes, acting downstream from PKCδ, is responsible for the phosphorylation. Another approach to determining the role of PKCδ involves the identification of interacting proteins using the yeast two hybrid screen. The sequence of nine out of ten independently isolated clones from a two hybrid screen showed perfect homology to human ribosomal protein L8. This protein has previously been shown to exist in complexes with ribosomal RNA, aminoacyl-tRNA and elongation factor-1α, a known substrate of PKCδ, suggesting a role for PKCδ in protein synthesis regulation.