Run coding applied to the digitized video signal from a TV scan of cell preparations can effect a substantial reduction in the total amount of data, sufficient to permit a moderate size of store to be loaded within one frame time with a representation of the field adequate for computer analysis. This paper describes the design of a run coding interface between a TV scanner and a computer store which also allows control of scan domain, spatial resolution and density resolution. Results are presented showing its efficiency when dealing with cervical smear preparations.