Linear density gradients were used to separate either unsensitized human peripheral blood lymphocytes or cells sensitized in mixed leukocyte culture (MLC), at different time points in the immune response. Testing of cells from the various density fractions for their ability to respond in MLC and cell-mediated lympholysis (CML) revealed that (a) activity between individual fractions differed, as well as activity between individual fractions and a suspension of unfractionated cells, (b) although precursor cells of MLC and CML were maximally enriched in closely associated light density regions, their respective distribution profiles were strikingly different, and (c) the density of cells responding in MLC and CML differed depending upon the point of the immune response at which the cells were separated. Whereas effector cells were of light density relative to the precursor cells, putative memory cells for CML exhibited a high density.