The free-living nematode Panagrellus redivivus can be used as a biochemical model for parasitic nematodes in the search for new chemotherapeutic agents. A novel cystathionine beta-synthase has been purified 3600-fold from the cytosol of P. redivivus. The enzyme catalyses the synthesis of cystathionine from homocysteine plus serine or cysteine. The enzyme, native M(r) 71.7 kDa, pI 4.7, is a dimer and also catalyses the replacement of the beta-SH group of cysteine with 2-mercaptoethanol to yield a thioether, S-(2-hydroxyethyl) cysteine and H2S. This reaction proceeds much faster than cystathionine synthesis and L-cysteine cannot be replaced by D-cysteine, L-cystine, N-acetyl L-cysteine, cysteamine of D,L-homocysteine. 2-Mercaptoethanol in the assay can be replaced by monothiolglycerol and to a lesser extent by cysteamine. The absolute K(m) values for L-cysteine and 2-mercaptoethanol were 0.13 +/- 0.05 mM and 1.72 +/- 0.24 mM, respectively, the absolute V(max) was 55 +/- 4.9 mumol.min(-1).mg protein(-1). The enzyme had a pH optimum of approx. 8.5 and did not require metal ions for activity. The enzyme was inhibited by a series of substrate analogues, anthelmintics and plant phenols. The P. redivivus enzyme differs markedly from its mammalian equivalent and suggests distinctive differences in sulphur amino acid metabolism in nematodes.