Upon tryptic activation of pure human prophospholipase A2, a heptapeptide is released from the N-terminal part of the protein yielding active phospholipase A2 (EC 3.1.1.4). Both the kinetics of the activation process and the amino acid sequence of the activation peptide strongly resemble those of pancreatic zymogens of other mammalian sources. The kinetic properties of human phospholipase A2 and its zymogen are compared with those of the corresponding porcine enzyme using substrates present at micelles, molecular dispersed solutions or as monomolecular surface films. The most obvious difference between the human and porcine phospholipase A2 is the low enzyme activity of the former protein at pH 8.0 as compared to pH 6.0, both against micellar and monomeric substrates. Neither the Ca2+ binding properties nor the inhibition of the human enzyme using haloketones can easily explain this different pH optimum. The sequence analysis of the N-terminal region of the first 40 residues is reported.