Interleukin-2 (Il-2) producing human T-lymphocyte subsets were isolated by discontinuous density gradient centrifugation and their requirements for helper monocytes were investigated. The results show that T-lymphocytes from fractions 2, 3, and 4 from the density gradient (high density lymphocytes) which contain up to 75% Fcmu receptor-bearing cells, and less than 0.2% monocytes could be stimulated by PHA for the production of Il-2 only in the presence of additional monocytes. T-lymphocytes from fractions 6 and 7 and in most experiments also from fraction 5 (low-density lymphocytes containing between 20% and 50% Fcmu receptor bearing cells, but still 0.2% up to 30% monocytes) could be stimulated by PHA for the production of Il-2 also without additional monocytes. Dose-response experiments with various numbers of monocytes added to the system showed the following: the higher the density of a cell population, the higher the number of monocytes required for help in the stimulation of Il-2 production. However, the production of Il-2 by high-density lymphocytes did not reach the Il-2 production by low-density T-lymphocytes even in the presence of high numbers of additional monocytes. Dose-response experiments indicated that the difference in Il-2 producing activity of T-lymphocytes of different density cannot be explained by the various numbers and/or activities of residual monocytes present in the various T-cell fractions. We suggest that the various T-lymphocyte populations differ with respect to a further cell population, presumably of lymphatic origin. This population appears to participate in the regulation of Il-2 production.