Abstract: The viability of a pack of stored blood platelets is monitored by gripping the pack between two plates (6) and (7) which are closed to pinch together the walls of the bag (10) along an L-shaped seal (14), leaving a channel (17), squeezing a part (15) of the bag by means of a reciprocating plunger (19), so that the platelets continually flow to and fro between the part (15) and the part (16), through the channel (17), and passing a beam of light from an LED (27) through the channel (17), to a photoresistor (29). The AC signal from the photoresistor corresponds to the fluctuations in the intensity of the light passing through the channel (17) and the amplitude of this signal is representative of the viability, and hence the clinical acceptability, of the platelets.