In this work Sf-9 cells previously adapted to SF900II and EXCELL 401 serum-free media (SFM) were grown and infected at different agitation rates in order to study the effects of power input upon cell growth, infection and production of Pr55gag particles in both SFM. Maximum cell concentration increased from 3 x 10(6) to 6 x 10(6) cells cm(-3) and 6.5 x 10(6) cells cm-3 when the agitation increased from 80 to 250 rpm or sparged aeration (0.01 vvm at 170 rpm) is used, clearly indicating that cell growth is limited by gas transfer. The specific productivity increased 3.5-fold when the agitation rate was increased from 80 to 170 rpm, indicating that in SFM cell infection is also limited by gas transfer. The highest product concentration was obtained in SF900II at 120 h post-infection (hpi). The product quality analysis showed that SF900II is the best medium for production of Pr55gag particles and that a careful optimisation of the harvest time is required. The maximum product titre was obtained at 120 hpi, 48 h after the achievement of the highest quality.