Abstract: This invention provides methods of increasing the frequency of contact between vectors and stationary target cells in an apparatus containing them which involves causing the vectors to move towards the target cells with motion above and beyond random Brownian motion. The methods of this invention include causing the vectors to move in the direction of the cells by (1) causing flow-through of a liquid containing the vectors through or past a cell bed, (2) moving charged vectors towards the target cells by electrodiffusion and (3) centrifuging vectors and cells to cause settling of vectors onto the cells.