Using a nonengineered Trichoplusia ni insect cell line, Tn-4s, infected with an Autographa californica recombinant baculovirus, 20% sialylation of human secreted placental alkaline phosphatase (SEAP) was observed. In contrast to this level of sialylation, intermediate complex forms with terminal galactose or N-acetylglucosamine were found in low proportions (<3% and <1%, respectively). We tested whether time of harvest or degradation of intermediate complex forms is responsible for this distribution of glycoforms. Spinner-flask cultures were infected with the SEAP baculovirus expression vector, and the cultures were harvested 48, 72, and 96 h post-infection. Structural analysis revealed that the glycoform distribution of SEAP was very similar at the different times of harvest, indicating that the cellular machinery was not significantly affected by the progress of infection and that the glycoforms obtained were stable. High levels of beta-galactosidase and N-acetylglucosaminidase activity were detected throughout infection. In contrast, sialidase activity was below detection level both in cell extracts and in supernatants. These levels of glycosidases activities raise the possibility that intermediate complex glycoforms may be degraded while sialylated forms should not experience significant degradation in this cell line. However, culture in the presence of extracellular beta-galactosidase and N-acetylglucosaminidase inhibitors did not significantly improve glycosylation, suggesting that extracellular degradation processes are not taking place. Instead, results suggest that the intracellular machinery of the Tn-4s cells tends to either shunt the glycans to paucimannosidic forms or drive them completely to sialylation.