Polyamines are mainly transported in the blood by erythrocytes: Putrescine, spermidine and spermine can be taken up in vitro by red blood cells (RBC); their entry is greater in the presence of serum than in the presence of plasma, and spermine entry is lower than that observed for the two other polyamines. In the presence of serum, the affinity of RBC for spermidine is 30 fold greater than that for putrescine. The majority of RBC polyamines are present in the hemolysate and are not complexed to high molecular weight material. At + 4 degrees C the polyamine uptake is considerably reduced and for putrescine and spermine practically non existent, but it seems that it is internalization rather than binding which constitutes the dependent step. Though intracellular spermidine and spermine levels reflect differences in uptake rather than in outward flux across the cell membrane, the values of putrescine appear to be the resultant of influx and efflux. The presence of specific receptor sites for polyamines visualized by SEM on the surface of RBC using latex-putrescine spheres, confirms the results obtained with labelled polyamines. Therefore, only the understanding of the polyamine repartition inside the blood compartments would permit the clinical use of those molecules as non statistical tumor markers.