Solutions of lysozyme in heavy water were studied by small-angle neutron scattering (SANS) at concentrations of 40, 20 and 10 mg ml^-1 with and without the addition of precipitant, and at temperatures of 10, 20 and 30°C. In addition to the expected protein monomers, dimeric and octameric species were identified in solutions at the maximum concentration and close to the optimal conditions for crystallization. An optimal temperature for octamer formation was identified and both deviation from this temperature and a reduction in protein concentration led to a significant decrease in the volume fractions of octamers detected. In the absence of precipitant, only monomers and a minor fraction of dimers are present in solution.Solutions of lysozyme in heavy water with added precipitants were studied by small-angle neutron scattering at lysozyme concentrations of 40, 20 and 10 mg ml^-1 and at temperatures of 10, 20 and 30°C. In addition to protein monomers, dimeric and octameric oligomers were also found in solution at the maximal concentration near the optimal crystallization conditions. The volume fraction of octamers decreases on deviation from the optimal temperature and on decreasing the protein concentration. In the absence of the precipitant, only monomers and a minor fraction of dimers are present in solution.