Twenty patients with sickle cell disease (14 black females and 6 black males, mean age 31.5 +/- 9.3) were studied by quantitating peripheral T and B lymphocyte percentages and measuring lymphocyte blast transformation (LBT) in response to phytohemagglutinin-P, concanavalin-A and pokeweed mitogen. Compared to normal black controls (19 black females and 1 black male, mean age 32.0 +/- 9.2 years) sickle cell patients had decreased T lymphocytes (50.2 percent +/- 6.2 compare; to 66.2 percent +/- 1.7) and increased B lymphocytes (17.0 percent +/- 3.4 compared to 7.7 percent +/- 1.1). Sickle patients exhibited decreased LBT to all three mitogens.