The properties and cytotoxicity of aqueous colloidal solutions of cadmium sulfide CdS nanoparticles obtained by chemical condensation were explored. Dynamic light scattering and optical spectroscopy were used to study the hydrodynamic diameter, size distribution, zeta-potential and optical properties of the CdS nanoparticles in solution. The cytotoxicity of colloidal CdS nanoparticles at different concentrations was assessed on cultures of human dermal fibroblasts and the cancer HeLa line.