Lamin B receptor (LBR), a nuclear protein of avian and mammalian cells, contains an hydrophobic domain that shares extensive structural similarities with the members of the sterol reductase family. To test if the sterol-reductase-like domain of LBR could be enzymatically competent, several sterol reductase-defective strains of Saccharomyces cerevisiae were transformed with a human-LBR expressing vector. LBR production did not change the ergosterol biosynthesis defect in an erg4 mutant impaired in sterol C24(28) reductase. In contrast, the sterol C14 reduction step and ergosterol prototrophy were restored in LBR-producing erg24 transformants which lack endogenous sterol C14 reductase. To test the effects of C14 reductase inhibitors on LBR activity, we constructed EMY54, an ergosterol-requiring strain that is devoid of both sterol C8-C7 isomerase and sterol C14 reductase activities. EMY54 cells recovered the capability of synthesizing ergost-8-en-3beta-ol upon transformation with a vector that expressed either yeast sterol C14 reductase or hLBR. In addition, growth in sterol-free medium was restored in these transformants. Sterol biosynthesis and proliferation of LBR-producing cells were found to be highly susceptible to fenpropimorph and tridemorph, but only moderately susceptible to SR 31747. Our results strongly suggest that hLBR is a sterol C14 reductase.