This study investigates the ability of P388D1 macrophages to synthesize and secrete substance P (SP). Using a monoclonal anti-SP antibody (termed MASP-1) coupled to Sepharose, it was possible to immunoaffinity purify from culture supernates a peptide that was antigenically related to SP. P388D1 macrophage cultured in the presence of 35S-methionine secreted into culture supernates a labelled peptide which could be recognized by MASP-1. Affinity-purified, P388D1-derived SP was shown to be chemically similar to synthetic SP using gel-filtration chromatography and reverse-phase HPLC. In addition, an RIA using a polyclonal, monospecific antibody was used to quantify the amount of secreted SP in cultured supernates. P388D1 macrophages secreted 222 pg SP per 10 (8) cells, whereas SP secretion by control thymocyte cultures was not detectable. The functionality of the P388D1-derived SP was also investigated. Since exogenously added SP can increase secretion of an interleukin-1 (IL-1)-like activity in these cells, we questioned whether an anti-SP antibody could remove P388D1-secreted SP from the culture, and in turn reduce cytokine production. By culturing varying dilutions of MASP-1 with P388D1 cells, it was possible to decrease cytokine production by P388D1 cells compared to cultures containing no antibody or with normal mouse immunoglobulin G (IgG). Taken together, these studies demonstrate that macrophage-derived SP may function in an autocrine or paracrine fashion to modulate macrophage function.