Human blood lymphocytes can be separated into two populations according to the presence of surface complement receptors. Lymphocytes containing complement receptors (CR+) were found to have a high rate of RNA synthesis or turnover accompanied by increased protein synthesis. Lymphocytes not containing complement receptors (CR-) while maintaining a low profile in RNA synthesis, had a 10-12-fold greater activity in adenosine deaminase enzyme which is believed to be related to lymphocyte-immune responses and cell-mediated immunity. These two biochemical characteristics can be useful tools for future studies in lymphocyte functions. By using these two biochemical markers, we found that CLL lymphocytes were predominantly of the CR+ type, had high active RNA synthesis, and very low adenosine deaminase level. Lymphocytes from two patients with X-linked agammaglobulinaemia showed a picture opposite to that of CLL.