Peripheral lymphocytes from normal individuals and from patients with chronic lymphocytic leukemia (CLL) were cultured in vitro for 1–7 days. The growth response to phytohemagglutinin (PHA) was quantitated by the incorporation of tritiated uridine into RNA nucleotide during a 2-hr pulse with the radioisotope. While the maximum response in PHA- stimulated normal cultures appeared at 2–3 days, CLL cultures required 5–7 days to develop their maximal response, which was 50%-60% of the normal magnitude. Dilution of the number of normally reactive lymphocytes by culturing them with totally unreactive, mitomycin- treated cells produced a normal 72-hr maximal response, no matter what proportion of unreactive cells was included in the PHA-stimulated cultures. In addition, the response of peripheral lymphocytes from patients with myeloblastic leukemia, where large numbers of unreactive myeloblasts diluted the normal small lymphocytes, a depressed reaction occurred at the anticipated 2–3 days. Nylon fiber-adherent lymphocytes consisting of 85% immunoglobulin (Ig)-bearing cells responded minimally to PHA, but showed no evidence of a delay. When isolated from CLL patients, both fiber-adherent cells (Ig-bearing) as well as non-fiber- adherent (sheep erythrocyterosetting) cells responded to PHA in a delayed fashion. Similarly, a case of CLL, in which 93.5% of the circulating lymphocytes bore sheep red blood cell receptors, showed its peak response to PHA at 7 days. Therefore, using surface marker criteria considered characteristic of normal T cells and B cells, the delayed response to PHA on the part of CLL lymphocytes was independent of thymic or nonthymic origin.