PbS nanocrystals (NCs) are synthesized in aqueous phase within a temperature range of 40-80 degrees C in the presence of native and denatured states of bovine serum albumen (BSA) as the capping/stabilizing agent The NCs are characterized with the help of field-emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, and energy-dispersive X-ray analysis. At 40 degrees C, large ball-shaped NCs (145 +/- 37 nm) with small surface protrusions are formed when 1 x 10(-4) g mL(-1) BSA is used. As the reaction temperature is increased towards 80 degrees C, the size of NCs decreases and they acquire somewhat cubic geometries (49.1 +/- 7.0 nm) due to a change in the capping behavior of BSA between its native and denatured states. The native and denatured states of BSA are simultaneously studied by fluorescence spectroscopy using tryptophan emission, and pH measurements with respect to time and temperature. Gel electrophoresis is used to determine the polarity of the BSA capped NCs. Only the small sized NCs conjugated with relatively larger amounts of BSA show a displacement towards the positively charged electrode in comparison to larger NCs with lower amounts of BSA capping. It was concluded that the denatured state of BSA is more effective in controlling the crystal growth of PbS than its native state especially in the low concentration range.