A multifunctional reagent for erythrocytes containing an amount sufficient to produce the lysis of erythrocytes or the sphering of erythrocytes in such a way that they can be detected by a cytometer or an automatic counting device, of a carbamate or of an agent inducing the formation by the erythrocytes, from carbonate and from a nitrogenated heterocycle or ammonium ions, of a carbamate combined with the absorption of CO.sub.2 by the erythrocytes, process for lysing or sphering erythrocytes and preparation process for leucocytes.