Oligomers of (R)-3-hydroxybutanoate (OHB) have previously been shown to generate nonselective single-channel conductance through planar phospholipid bilayers in patch-clamp experiments (voltage induced). In view of the ubiquitous occurrence of low molecular weight, complexed polyhydroxybutanoate in essentially all living organisms, we have now studied the effect of OHBs on the Ca2+ ion permeability through vesicle bilayers (concentration-gradient induced). Thus, 8-, 16-, 32- and 64-mers of HE with and without O-terminal coumarin fluorescence label were incorporated in Quin-2-encapsulating vesicles (ca. 200 nm diameter) by sonication. The physiologically most important Ca2+ transport into the vesicles was studied by following the Ca2+-dependent Quin-2 absorption at 264 nm. There was no or erratic Ca2+ transport detectable with the 8- and 16-mer, whereas the longer-chain OHBs tested (32- and 64-mers) showed a transport behavior with turnover numbers in the range of 2-3 s(-1), very similar to calcimycin. Furthermore, the OHB-mediated Ca2+ transport was inhibited by the presence of La3+. A carrier-like mechanism is suggested, involving three OHB molecules in the 32-mer, and Ca2+ ions hopping between complexing carbonyl groups. (A channel mechanism with much higher turnover numbers is operative in the corresponding patch-clamp experiments.)