Immunological methods have been developed for quantitation of a protein, provisionally called L1, which constitutes about 5% of the total protein in granulocytes. The protein has a mol wt of about 51000 daltons and an isoelectric point of about 6.3–6.5. It is readily released from leucocytes during storage and following immunological injury to the cell membrane. The concentration of L1 in normal plasma is about 300 ng/ml, but levels above the normal mean + 5 SD were found in 23 out of 30 patients with various malignant diseases and in 4 patients with septicaemia. It is suggested that plasma levels of L1 may reflect the rate of leucocyte turnover in vivo and that the L1 release phenomenon may be used for studies on leucocyte reactions in vitro.

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