An acid glutathione S-transferase from human liver has been partially purified and characterized. The relative molecular mass of the enzyme is 46,000, and a double reciprocal plot of velocity against glutathione concentration is biphasic and shows in addition substrate inhibition. The enzyme differs from the basic human liver transferases alpha, beta, gamma, delta, and epsilon in the characteristics studied, but it bears a resemblance to transferase rho from human erythrocytes. When liver cytosol was analysed by isoelectric focusing using a short pH gradient and a density gradient formed of either glycerol or saccharose, the peak of glutathione S-transferase activity appeared at pH 4.63 +/- 0.02, in contrast to blood cell lysate which was found to contain a major peak at pH 4.63 and at least two additional peaks at pH 4.44 and 4.51, respectively.