We examined the sphingolipid metabolism of peripheral blood cells, i.e. platelets, erythrocytes, neutrophils and mononuclear cells. A distinguishing characteristic of sphingolipid metabolism in these highly differentiated cells was their high sphingosine (Sph) kinase activity. The occurrence of [³H]sphingosine 1‐phosphate (Sph‐1‐P) from [³H]Sph (actively incorporated from the outside) in the blood cells was strong, long‐lasting, and independent of cell activation. Hence, the possibility of Sph‐1‐P playing a second messenger role is remote in these cells. About 40% of platelet Sph‐1‐P could be released extracellularly by 12‐O‐tetradecanoylphorbol 13‐acetate, possibly through mediation by protein kinase C. On the other hand, in erythrocytes, neutrophils and mononuclear cells a significant percentage of Sph‐1‐P formed inside the cell was discharged without stimulation, whereas the stimulation‐dependent release was marginal. In contrast to active [³H]Sph conversion to [³H]Sph‐1‐P, formation of [³H]sphingomyelin was barely detectable in the blood cells; this was especially true for anucleate platelets and erythrocytes. The Sph → Sph‐1‐P pathway may become predominant over the Sph → Cer → sphingomyelin pathway during late‐stage differentiation into platelets or erythrocytes. Sph and its methylated derivative, N,N‐dimethylsphingosine, induced apoptosis not only in neutrophils but also in mononuclear cells, whereas Sph‐1‐P elicited Ca²⁺ mobilization in platelets. Our results suggest that all blood cells may remove plasma Sph, which is harmful or suppressive to cellular functions, and change it into Sph‐1‐P, acting as the source of plasma Sph‐1‐P, which may play a variety of important roles in blood vessels.