Experiments were designed to study the way in which normal human polymorphs (PMNs) inhibit the production or release of colony stimulating activity (CSA) by normal lymphocytes incubated alone or in conjunction with normal monocytes. PMNs were first incubated with lymphocytes for various periods at various concentrations. The PMNs were then removed and the 'conditioned' lymphocytes were used alone or after addition to adherent mononuclear cells (monocytes) for the production of conditioned medium. The samples of conditioned medium were then assayed for CSA in a standard system for culturing granulocyte/macrophage progenitor cells (CFU-C) in agar. We found that the capacity of PMNs to inhibit CSA production by lymphocytes or by lymphocytes plus monocytes was directly proportional to the number of PMNs originally incubated and maximal at relatively short incubation times (i.e. 2-4 h). Such inhibition could be counteracted by the introduction of known stimulators of CSA production by monocytes, e.g. phytohaemagglutinin or bacterial toxin. We conclude that normal lymphocytes 'conditioned' or 'programmed' by contact with PMNs may themselves have a reduced capacity to produce CSA and may also act to reduce CSA production by monocytes. Such programmed lymphocytes could thus be a component of the mechanism by which PMNs exert a physiological inhibition on CSA-dependent granulopoiesis in vivo.