The invention concerns a reagent for determination of leucocytes and measurement of haemoglobin in a sample of blood. This reagent comprises a buffer system that is suited to adjust selectively the pH of the reagent to an acidic value, at least one detergent of cationic type, a nitrogenous compound and, optionally, at least one inorganic salt. This reagent can be used in haematological analyses in human medicine and also permits the identification of a leucocytic subpopulation, in particular the basophil polymorphonuclear leucocytes.