A stackable cassette or rack for supporting a plurality of sealed sample containers having different diameters and/or lengths and transporting the same to a testing station of a hematology analyzer device. The cassette comprises a body having a top and a bottom and a rear portion, and the body includes a base, front and intermediate walls and a biasing means, connected between the front and intermediate walls, both walls longitudinally extend across the base and each wall has a plurality of equidistantly spaced openings therein arranged in a row lengthwise of the cassette which openings extend from the base to the top of the body. Each of the openings in the front wall has an upper edge and the spaced apart holes of both walls are in alignment. The biasing means are connected to the bottom of the body to provide secure and proper positioning for all sample containers inserted through the spaced openings in the front and intermediate walls and so that their upper walls abut against the upper edges of the openings in the front wall, the later to facilitate accurate optical reading of information placed on their upper wall surfaces. The biasing means also maintains a parallel orientation between the longitudinal axis' of the inserted sample containers and their receptacles. Further included is a plurality of equidistantly spaced channels arranged in a row lengthwise of the cassette and extending from the base to the top of the body and from the intermediate wall to the end of the rear portion of the body and open ended thereat and arranged so as to permit entry of a rod member.