Glycosylation is acellular process accomplished by a series of sequential enzymatic processing steps through the endoplasmic reticulum and Golgi apparatus with vesicle transport between the membranous organelles. The capacity for complex glycosylation is considered to be conferred by cell genetics, while the roles of nongenetic factors in protein processing are often ignored. It was hypothesized that the glycosyltransferase reactions in the insect cell-baculovirus system were limited by the small supply of sugar donor cosubstrates. By adding mannosamine, the,glycosylation of a human secreted alkaline phosphatase in Spodoptera frugiperda (Sf-21) cells was extended to include terminal N-acetylglucosamine structures which were not seen in control cultures, and in Trichoplusic ni (BTI-Tn5B1-4) cells the amount of terminal N-acetylglucosamine structures was increased.