Apolipoprotein C-II (apoC-II) is an exchangeable plasma apolipoprotein and an endogenous activator of lipoprotein lipase (LpL). Genetic deficiencies of apoC-II and overexpression of apoC-II in transgenic mice are both associated with severe hyperlipidemia, indicating a complex role for apoC-II in the regulation of blood lipid levels. ApoC-II exerts no effect on the activity of LpL for soluble substrates, suggesting that activation occurs via the formation of a lipid-bound complex. We have synthesized a peptide corresponding to amino acid residues 39-62 of mature human apoC-II. This peptide does not bind to model lipid surfaces but retains the ability to activate LpL. Conjugation of the fluorophore 7-nitrobenz-2-oxa-1,3-diazole (NBD) to the N-terminal alpha-amino group of apoC-II39-62 facilitated determination of the affinity of the peptide for LpL using fluorescence anisotropy measurements. The dissociation constant describing this interaction was 0.23 microM, and was unchanged when LpL was lipid-bound. Competitive binding studies showed that apoC-II39-62 and full-length apoC-II exhibited the same affinity for LpL in aqueous solution, whereas the affinity for full-length apoC-II was increased at least 1 order of magnitude in the presence of lipid. We suggest that while the binding of apoC-II to the lipid surface promotes the formation of a high-affinity complex of apoC-II and LpL, activation occurs via direct helix-helix interactions between apoC-II39-62 and the loop covering the active site of LpL.