Modifications ofthe Technicon S.M.A.4 and S.M.A.4A equipment which substitute automated platelet counting for red cell counting are described. Experimental results are presented which establish the validity and reproducibility of the method and show that the results are quantitatively comparable to those obtained by manual and semi-automated techniques. Experience of the method in routine use for approximately 3 yr, during which time about 150000 platelet counts have been done, indicates that the modification produces no increase in maintenance or running problems.Automation of cell counting and haemoglobin determination is now a well-established practice. To date, evaluation of the more fully automated methods, using Coulter or Technicon S.M.A.4/7 equipment, has been principally concerned with red and white cell counting rather than with platelet counting (Rosenfield et al, 1966; Brittin et ul, 1969; Green et al,
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