For use in automated or semi-automated hematology analysis apparatus, a method of introducing a volume of blood from a source (c) thereof along a conduit (14, 16) leading to sampling, metering and transfer valve assembly (12) of the apparatus comprising the steps of drawing a predetermined volume of blood sample from the source (c), causing the drawn blood sample to travel along the conduit toward the valve assembly (12), providing detectors (18, 30) adjacent the inlet (28) and outlet (26) of the valve assembly and using vacuum to position the drawn blood sample so that it passes through the valve and is disposed in metering condition within the valve assembly (12) between the inlet and outlet, the positioning being responsive to signals provided by the detectors (18, 30).