Abstract: The invention concerns the use of hemoglobin derivatives for determining plasma and/or blood volume, wherein the hemoglobin derivatives are stable chromoproteins with a molecular weight of at least 64 kDa. The use can encompass the treatment of a patient wherein (a) a defined quantity of hemoglobin derivative is applied to a patient, (b) the mixing of hemoglobin derivative with the blood of the patient is awaited, (b) at least one sample is taken from the patient, (c) the content of the hemoglobin derivative and/or the content of the hemoglobin derivative bound ligand in the sample is determined and (d) the plasma and/or blood volume ascertained therefrom. The invention also concerns preparations for determining plasma and/or blood volume which contain hemoglobin derivatives and are characterized in that the hemoglobin derivatives are stable chromoproteins with a molecular weight of at least 64 kDa which carry at least in part carbon monoxide as ligand, as well as processes for their manufacture.