The bipolymer calf thymus deoxyribonucleic acid (DNA) is employed to stabilize cadmium sulfide crystallites in the quantum confinement size regime (Q-CdS). In this work, the synthesis and characterization of these semiconductor 'quantum dots' is described. These Q-CdS clusters are easily prepared in aqueous solution at room temperature and are extremely stable (for more than 17 months when stored at 5 degrees C). High-resolution transmission electron microscopy shows that the crystallites have an average diameter of 5.6 nm, with lattice images and diffraction patterns consistent with the zinc-blende structure of CdS. For approximately 15% of the particles, unique hollow-sphere- or hollow-hemisphere-shaped CdS structures are observed, and their presence attributed to the influence of the DNA host. Spectroscopically, these clusters show an absorption edge blue-shifted from that of the bulk, consistent with quantum confinement, and broad trap emission characteristic of an appreciable number of defect sites at the semiconductor cluster interface, apparently induced in part by the host polynucleotide. The effects of the Q-CdS clusters on the macroscopic properties of the DNA are illustrated by the change in intrinsic viscosity upon addition of cadmium ions and subsequent CdS formation.