Abstract: A method for detecting the presence of cells or other target biological entities in a fluid sample, which method comprises: a) contacting the sample with a specific binding partner having (i) an electrophoretic, or zeta potential, label or (ii) a fluorescence label; b) allowing the specific binding partner to bind to any said cell or other target biological entity present in the fluid sample; c) in an electric field measuring the value of the velocity, displacement, zeta potential or electrophoretic mobility of any said cell or other target biological entity present in the fluid sample and that is bound to the specific binding partner; and d) observing the value obtained in step c) as indicative of the presence of a said cell or other target biological entity in the fluid samples.