Chick embryo cell cultures were examined at intervals after infection with avian myeloblastosis virus (AMV) for virus-specific complement fixing antigen, both associated with the cells and in the culture fluids. Most of the cell-associated antigen was released by dilute trypsin. The antigen of the culture fluid had the sedimentation properties on sucrose density gradients of the virion. After a 1–2 day latent period when no antigen was detected, both cell and culture fluid antigens increased rapidly, with an associated increase in the culture infectivity. A point on the growth curve (4 days) was selected when cell-associated antigens had reached a plateau and culture fluid antigens (virions) were produced at a constant rate. Then the incorporation of radioactive amino acids into protein and uridine into RNA was measured in both AMV-infected and control cultures after various periods of exposure to the precursors. The particles from the culture fluid and the trypsin-released fraction were sedimented, then were analyzed on sucrose density gradients. Particles were released by trypsin from AMV-infected cells that, unlike the virions, were sensitive to RNase. No similar particles were released from control cells, even unlabeled virus-infected cells exposed to labeled virus. The trypsin-released particles had incorporated 14C-amino acids after only a brief exposure, suggesting that at least a portion of the virus protein is made at the cell surface. Accumulation of 14C-amino acid-labeled virions in the culture fluid proceeded more slowly, at the same rate as 14C-uridine labeling of the trypsin-released particles and culture fluid virions. These results imply that AMV is slowly assembled at the cell surface from protein and RNA subviral particulate precursors, which may be released prematurely by trypsin treatment.