Human cytomegalovirus (CMV) strain AD 169 was reacted with IgG antibody (ab) from a CMV-infected renal transplant patient. A portion of he virus was neutralized, but infectious CMV-ab complexes that could be neutralized by adding rabbit anti-human IgG (A-IgG) or complement (C) were also generated. The immune complexes were examined, and the following observations were made: 1) CMV ab sufficient to cause 94% neutralization did not induce measurable changes in virion size or density. 2) The CMV-ab complexes increased slightly in size after reaction with A-IgG. 3) C increased the size and density of CMV-ab complexes to a greater degree than A-IgG. Virus aggregation did not occur with ab alone or with ab + A-IgG. However, clumping may have occurred in the presence of ab + C. 4) C also damaged virus envelopes, rendering viral DNA sensitive to DNase. 5) CMV-ab complexes absorbed to host cells as efficiently as native CMV. A-IgG or C partially inhibited complex attachment to the cells and increased the rate of release of cell-attached CMV. These findings suggest that virus neutralization may occur in a multistage process by more than one mechanism depending on the immune reagents employed. The physical changes in virus particles caused by A-IgG or C may be contributing factors in the neutralization process.