Expression of certain mammalian protein kinase C (PKC) isoforms inhibits the proliferation of Schizosaccharomyces pombe (Goode et al., Mol. Biol. Cell 5 (1994) 907-920). We have taken advantage of this fact to determine the in vivo isoform preference of a number of PKC inhibitors, using a microtitre plate assay which allows rapid screening. This in vivo model has revealed previously unreported preferences; calphostin C is a more efficient inhibitor of the novel PKCS than chelerythrine chloride whereas the efficiencies are reversed for inhibition of the classical PKCgamma. We have also shown that the anti-leukaemic agent bryostatin 1 inhibits or activates in vivo in an isoform-specific manner.