The binding characteristics of two monoclonal antibodies (mAb) to phosphatidylinositol-4-phosphate (PIP) were examined: a murine IgM mAb to PIP; and a human IgG mAb (4E10) that binds both to HIV-1 envelope protein and also to neutral and anionic phospholipids, including PIP. Binding of each mAb to pure PIP was inhibited by Ca2+ as determined by ELISA. When studied by surface plasmon resonance, liposomes containing PIP could be stripped (i.e., removed) by either Ca2+ or phosphorylated haptens after binding of the liposomes to the murine anti-PIP antibody attached to a BIAcore chip. In contrast, the binding of liposomal PIP to 4E10 was irreversible and could not be stripped. We therefore conclude that Ca2+ and phosphate can modulate the initial binding of both types of antibodies to PIP. However, 4E10 binds to liposomal PIP in a two-stage process involving first Ca2+-modulated binding to the PIP polar head-group, followed by irreversible binding to liposomal hydrophobic groups. Published by Elsevier Inc.