Isoforms of porcine pancreatic phospholipase A2 (PLA2) can be differentially regulated by heparin. The major isoform of PLA2 can bind to heparin-Affigel and its catalytic activity can be inhibited by heparin. The interaction between this PLA2 isoform and heparin does not require calcium ion or a functional active site. The sensitivity to heparin inhibition depends on the pH, with optimum sensitivity at pH 5-7 and greatly diminished sensitivity as the pH is increased from 7 to 10. A minor isoform of porcine pancreatic PLA2 cannot bind to heparin and is resistant to heparin inhibition. The resistant isoform appears to be iso-pig PLA2. Heparin affinity chromatography therefore offers a convenient route to the isolation of structurally and functionally distinct classes of PLA2 enzymes. The existence of classes of PLA2 that can be differentially regulated by heparin may have important physiological consequences.