The low-temperature photoluminescence (PL) measurements (down to 4.2K) were employed for the investigations of the defects and impurities in multicrystalline silicon (me-Si) samples grown by block-casting method. The optical properties of as-grown, irradiated by gamma-rays, heat and hydrogen plasma treated samples were studied. It was found that carbon and oxygen as the residual impurity atoms are responsible for the formation of the zero-phonon PL lines with 0.9355 eV (T line) and 0.9652 eV (I line) after heat treatments at about 350-550degreesC. The appearance of PL lines with the energies of 0.9697 eV (A line) and 0.7894 eV (C line) after a gamma-rays irradiation can be attributed to the formation of carbon- and oxygen-related centers, respectively. The comparison of the PL properties of the mc-Si samples with the mono-crystalline one is performed, It is shown that the main peculiarities of the low-temperature PL spectra of me-Si can be explained both by the influence of residual impurities and the residual strains in this material. (C) 2002 Elsevier Science B.V. All rights reserved.