The effects of poly(vinyl sulphate) (PVS), poly(vinyl pyrrolidone) (PVP) and protamine sulphate with the enzyme pepsin (EC 3.4.23.1) have been investigated. PVS, PVP and protamine acted as inhibitors of the enzyme pepsin at low concentrations, but at high concentrations of the polyions (with the exception of PVS) the inhibition was less pronounced. The catalytic effectiveness of several polyions has been shown experimentally; when used at pH 2.1 with haemoglobin and at pH 5.0 with azocasein thus acted as a weak proteolytic catalyst. The order of effectiveness was: polybrene greater than poly(L-lysine) (PLL) greater than spermine greater than spermidine greater than protamine greater than and PVP relative to pepsin. The hydrolysis of the substrates by the enzyme decreases in the presence of high concentrations of monovalent and/or divalent salts. The purity of the enzyme was assessed by determination of the molecular weight using sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), phosphorus and nitrogen content. The circular dichroism (c.d.) spectrum of the enzyme in the wavelength range 240-310 nm, where the polycations did not have a c.d. spectrum has also been studied. Each of the polyions had a definite effect on the c.d. spectrum, showing strong binding to the enzyme. The neutral polymer PVP was also found to modify the c.d. spectrum, showing strong binding to the enzyme. The strengths of the interactions as indicated by the magnitudes of the changes in the c.d. spectrum of pepsin at 270 nm were: polybrene greater than protamine greater than PLL greater than PVP greater than spermidine greater than spermine.