A vector system has been developed to express isoenzyme Al of sweet potato peroxidase (POD) and was introduced into Saccharomyces cerevisiae. The system contains the signal sequence of Aspergillus oryzae alpha-amylase to facilitate the extracellular secretion of peroxidase under the control of constitutive glyceraldehyde-3-phosphate dehydrogenase (GPD) promoter. In a batch culture using YNBDCA medium (yeast nitrogen base without amino acids 6.7 g l(-1), Casamino acids 5 g l(-1) and glucose 20 g l(-1)), the recombinant strain expressed the swpa1 gene giving a secretion yield of POD activity of ca. 90% of total expressed peroxidase. Supplementation with PMSF (0.05 mM) and Casamino acids (5 g/50 ml) increased extracellular POD activity to nearly 10 kU ml(-1), equivalent to 1.5 kU g(-1) cell dry wt. This is 9 fold higher than that obtained in medium without PMSF. From SDS-PAGE and native-PAGE analyses POD has an M-r of 53 kDa.