Dedicated to Prof. Dr. Edgar Heilbronner on the occasion of his 75th birthday (30.X.95) Monodisperse and polydisperse oligomers and polymers of 3-hydroxybutanoic acid (3-HB) containing 8, 16, ca. 28,32, ca. 60,64,96, and ca. 3000 monomer units were incorporated into palmitoyl-oleoyl-phosphatidyl choline (POPC) planar bilayers. At concentrations of 0.1-5% of oligo(3-HB), the resulting phospholipid bilayers showed typical single-channel behavior for Rb+ and Ba2+ ions, using the patch clamp technique. Thus, channel-forming activity of a pure polyester has been demonstrated for the first time (Figs. I, 3, and 6 ) . Single-channel activity depends upon the following structural parameters of the 3-HB derivatives: unprotected OH and COOH groups on the chain ends; chain length 2 16 monomer units; no high-molecular-weight as in P(3-HB). The results are discussed in view of the Ca2+-specific channel formed with the P(3-HP)/Ca .PP, complex from genetically competent Escherichiu coli and in view of the ubiquitous occurrence of low-molecular-weight P(3-HB) in prokaryotic and eukaryotic organisms. A simple model for the channel-causing structure is proposed, based on the proven tendency of oligo-and poly(3-HB) to form ca. 50-8, thick lamellar crystallites.