Callosellasma rhodostoma (Malayan pitviper) is a monotypic Asian pitviper of medical importance. Three acidic phospholipases A 2 (PLA 2 s) and one basic PLA 2 -homolog were purified from its venom while 10 cDNAs encoding distinct PLA 2 s were cloned from venom glands of a Thailand specimen of this species. Complete amino-acid sequences of the purified PLA 2 s were successfully deduced from their cDNA sequences. Among the six un-translated PLA 2 cDNAs, two apparently result from recombination of its Lys49-PLA 2 gene with its Asp49-PLA 2 genes. The acidic PLA 2 s inhibit platelet-aggregation, while the noncatalytic PLA 2 -homolog induces local edema. This basic PLA 2 -homolog contains both Asp49 and other, unusual substitutions unique for the venom Lys49-PLA 2 subtype (e.g. Leu5, Trp6, Asn28 and Arg34). Three-dimensional modelling of the basic protein revealed a heparin-binding region, and an abnormal calcium-binding pocket, which may explain its low catalytic activity. Oxidation of up to six of its Met residues or coinjection with heparin reduced its edemainducing activity but methylation of its active site His48 did not. The distinct Arg/Lys-rich and Met-rich region at positions 10±36 of the PLA 2 homolog presumably are involved in its heparin-binding and the cell membraneinterference leading to edema and myotoxicity.