An improved optical flow cell adapted for use in a flow cytometer for differentiating formed bodies (e.g., blood cells) in liquid suspensions. Preferably manufactured by assembling, aligning, and optically joining at least two elements made from transparent material, the improved flow cell has a seamless internal flow channel of preferably non-circular cross-section in a cylindrical first element through which prepared samples can be metered and an independent second element having an external envelope suited to acquisition of optical parameters from formed bodies in such suspensions, the second element being conforming and alignable to the first element so that non-axisymmetric refractive effects on optical characterizing parameters of formed bodies passing through the flow channel in the first element may be minimized before the two elements are optically joined and fixed in working spatial relationship. Preferably, such first element is formed by a glass-drawing process in which a relatively large glass preform having a rectilinear internal channel of a desired cross-sectional shape is heated and drawn to achieve a desired cross-sectional area of reduced size. Preferably, such second element is provided by through-boring a suitable commercial optical component to conform to such first element. Also disclosed are preferred methods for differentiating formed bodies using the flow cell of the invention.