In a context of large-scale production of baculoviruses in serum-free media for use as gene delivery vectors, the stability of these viruses has become an important factor. The development of robust processes heavily relies on baculovirus stock stability. In the present work, we studied over a period of 300 days the stability of baculovirus vectors produced in serum-free media stored at 4, -20, or -80 degrees C or in liquid nitrogen. The viral stocks investigated were either crude baculovirus supernatant, baculovirus supernatant concentrated 10 times and diafiltered against fresh serum-free media by tangential flow filtration, or baculovirus purified by size exclusion chromatography. The results showed that baculovirus supernatant and dialiftered concentrate stored at 4 degrees C underwent a progressive loss of infectivity after a period of 100 and 50 days of storage, respectively. Aggregation has been recognized as the probable mechanism for the loss of infectivity. Baculovirus stocks were unstable at -20 degrees C, whereas in liquid nitrogen they retained infectivity after successive freeze thaw cycles. Concentration and diafiltration of baculovirus supernatant prior to storing at -80 degrees C contributed to improving viral stock stability over time. Glycerol as well as DMSO and sucrose have proven to be equally effective as additives to maintain the purified baculovirus stability after storage at -80 degrees C or in liquid nitrogen.