(b) QF-FRL of unilamellar liposomes containing 0-5 mol % of PtdIns(3) P. Scale bar, 0.2 μm. The labelling density increased roughly in proportion to the PtdIns(3) P content of the liposomes. The experiment was repeated twice and the number of colloidal gold particles per μm2 was measured in more than 75 randomly selected liposomes. Values for all concentrations are statistically different from those in the adjacent conditions (for example, 1 versus 2 mol%; mean±s.e.m.; Student's t-test; P0.05). 