Three adenovirus type 2-specified immunogens elicited neutralizing antibodies when injected into rabbits; these were the fiber, the hexon, and the penton base. Adenovirus type 2 virions, neutralized by antihexon- or anti-penton base antisera, attached to HeLa cells to the same extent as untreated control virus, and after attachment, neutralized viruses also became sensitive to DNase treatment. A fraction of 75 to 80% of the attached antibody-treated virions penetrated the plasma membrane, which should be compared with an 84 to 88% penetration level in the control series. A majority of the antihexon-neutralized virions was found in intracellular vesicles, as revealed with an electron microscope, but in the case of anti-penton base neutralization, a maximum of 50% of the virions was retained within vesicles, and ca. 30% was free in the cytoplasmic compartment. A value greater than 45% was never obtained for neutralization with a monospecific anti-penton base antiserum, which could imply the existence of alternative pathways for virus penetration into HeLa cells--one of these being sensitive to treatment with anti-penton base antiserum. Antisera containing antifiber specificities efficiently aggregated virions, and the aggregation data mirrored the degree of neutralization. Antifiber-neutralized virions attached to cells to a three- to five times greater extent than untreated control virus, but the former virions had a reduced ability to become sensitive to DNase treatment. Around 15% of the attached antifiber-treated virions was found as large aggregates inside multivesicular bodies or lysosomes.