A gene named stp1+, coding for a 17.5-kDa protein, that rescues cdc25-22 when overexpressed, has been previously isolated from fission yeast. Here we describe the expression and purification of Stp1 protein as a fusion with the glutathione S-transferase in E. coli and its kinetic characterisation. Stp1 deduced protein sequence shows an high homology to members of a class of cytosolic low M(r) protein phosphatase previously known to exist only in mammalian species. Stp1 has a kinetic behaviour that appears to be intermediate with respect to the two isoenzymatic forms of low M(r) protein tyrosine phosphatases present in mammalian tissues. These differing kinetic characteristics are mainly due to the sequence 45-56 that is spatially close to the active site pocket.