L-selectin is a glycoprotein which is one of three members in a family of cell adhesion molecules called selectins. L-selectin is present in distinct forms on both neutrophil granulocytes and lymphocytes, and it appears to play an important role in the early stages of leukocyte-endothelial cell interaction. Activation of leukocytes leads to shedding of the extracellular part of L-selectin which thus forms a soluble adhesion molecule, sL-selectin, which retains functional capacity and can be detected in serum. In the present study we have developed a specific, sensitive sandwich ELISA to measure the serum level of sL-selectin in patients with hematological and infectious disorders. Three patients with acute myeloid leukemia in remission and 1 patient with chronic myeloid leukemia in chronic phase were followed during bone marrow transplantation and the level of sL-selectin was found to correlate closely to the leukocyte counts with no detectable sL-selectin during periods of severe leukopenia. In 11 patients with chronic phase chronic myeloid leukemia and 13 patients with chronic lymphocytic leukemia the sL-selectin level was also found to correlate closely to the leukocyte count (R = 0.98; p = 0.001 and R = 0.83; p = 0.004 respectively). One CML patient with a leukocytosis of 385 x 10(9)/l was found to have an sL-selectin concentration 625 times above normal. Ten patients with acute pneumonia were evaluated at diagnosis and at the time of follow-up 4-8 weeks later. In all patients the initial sL-selectin level was higher than at follow-up. However, no close correlation between sL-selectin and leukocyte count or CRP (C-reactive protein) at the time of diagnosis was found. In summary, we have found that the sL-selectin level in human serum closely correlates to the leukocyte count in both CML and CLL and during bone marrow transplantation.