A problem often faced in clinical cell-mediated immunology research is to obtain peripheral blood lymphocytes (PBLs) in sufficient numbers to carry out cellular immune assays. Obtaining sufficient amounts of blood for serum is usually more practical than for PBLs. Blood is usually drawn easily for serum, but obtaining PBLs from clotted blood has not been very practical. In this report we describe a procedure whereby PBLs can be recovered from clotted blood and used in functional immune assays. Using the enzyme streptokinase (SK) blood clots were dissolved and viable PBLs recovered up to 24 h post clotting. Concentrations of SK from 250 to 500 SK U/ml gave maximum recovery of PBLs.