The activation of C55-isoprenoid alcohol phosphokinase by a variety of lipids has been investigated. A number of amphipathic lipids can serve as effective kinase activators. Both the nature of the polar and nonpolar groups are important, but kinase activation does not depend on any particular chemical structure or charge on the lipid. The structure of those lipids which are most effective, as well as an analysis of their temperature profiles, suggests that bulk physical properties are significant. Lipids which provide a hydrated, loosely packed, highly fluid environment are often effective activators.