Suspended KB cells can adsorb the majority of adenovirus particles in a suspension within 20 min. No detectable quantity of virus particles disappears within 1 hr after inoculation. Particles are still physically recognizable at 2 hr, but most have lost their infectivity. New virus appears at about 19 hr after inoculation and is formed at a uniform rate until about 54 hr. Ultimately, about 6700 particles and 67 50% tissue culture infectious doses (TCID50) are produced per infected KB cell. The formation of complement-fixing (CF) antigen roughly parallels the production of virus particles and infectious virus units. Over 98% of the countable virus particles and all of the measurable CF antigen remain intracellular throughout the growth cycle. Density gradients are sensitive indicators of new virus formation, viral bands being clearly visible early in the growth cycle. Particles from gradients having a density of 1.34 or higher are highly infectious (1 particle in 5 to 8 is infectious), whereas those having densities of less than 1.29 are practically noninfectious (only 1 particle in 500 to 1000 is infectious). Assembled or partially assembled virus particles account for no more than about 6% of the complement-fixing activity of crude, infected cell lysates. The remainder of the activity is in the form of very small, low density viral subunits. Purified virus having a density of 1.33 to 1.34 is not potent as CF antigen, more than 5 × 109 particles/ml being required for reaction. Ragged, partially assembled particles are somewhat more reactive, about 3 × 109 particles/ml being required to give a positive CF test. Artificially disrupted particles, showing a large number of capsomeres and a few short strands, are 4 to 8 times more potent as CF antigen than are intact particles. Electron microscopic examination of crude, infected cell lysates shows huge quantities of unassembled viral components. It is calculated that less than one half of the CF antigenic viral materials synthesized is finally assembled into physically recognizable virus particles. Only about 1% of these particles are completed sufficiently to be infectious. Therefore, the efficiency of assembling viral elements into infectious virus particles within the KB cell is probably less than 5 parts in 1000.