A study of the sensitivity of the integral properties of the plutonium-fueled fast-reactor critical assembly ZPR-6, Assembly 7 to uncertainties in the nuclear data has been performed with the aid of the VARI-1D variational sensitivity code; ENDF/B Version III was taken as a reference nuclear data base. The effect of 10% cross-section changes, which were correlated over the entire energy range, was analyzed, and allowed data uncertainties compatible with given uncertainties in integral parameters were established. Estimated uncertainties in important cross sections were considered, and the effect of different assumptions of the correlation in energy of the data uncertainties was examined. The effect of data changes among ENDF/B Versions I, II, and III was also analyzed. The most important data uncertainties are in the fission cross section of 239Pu and in the capture cross section of 238U, while the most important uncertainties in integral properties are in sodium void effect, breeding ratio, and keff. (The Doppler coefficient was not studied in the present work.)