The association between the cardiac transmembrane proteins phospholamban and sarcoplasmic reticulum Ca(2+) ATPase (SERCA2a) regulates the active transport of Ca(2+) into the sarcoplasmic reticulum (SR) lumen and controls the contraction and relaxation of the heart. Heart failure (HF) and cardiac hypertrophy have been linked to defects in Ca(2+) uptake by the cardiac SR and stimulation of calcium transport by modulation of the PLB-SERCA interaction is a potential therapy. This work is part of an effort to identify compounds that destabilise the PLB-SERCA interaction in well-defined membrane environments. It is shown that heparin-derived oligosaccharides (HDOs) interact with the cytoplasmic domain of PLB and consequently stimulate SERCA activity. These results indicate that the cytoplasmic domain of PLB is functionally important and could be a valid target for compounds with drug-like properties.