Immunoglobulins (Ig) were localized by immunofluorescence and immunoperoxidase techniques inside human milk macrophages (HMM phi). Immunoelectron microscopy was employed to study the distribution and localization of Ig within HMM phi. IgA and IgM were detected inside phagocytic vacuoles of different sizes in the vicinity of the cellular membrane and in the periphery of larger vacuoles with a dense and homogeneous content. The intramacrophagic IgA represents 5-10% of the total milk IgA. The in vitro release of IgA into the medium was quantitated using an enzyme-linked immunoabsorbent assay. The amount of IgA released was in most samples around 1,720-5,200 ng/10(6) HMM phi. The concentrations of IgM and IgG released by HMM phi were approximately 50-600 and 20-40 ng/10(6) HMM phi, respectively. The concentrations of Ig determined in the lysated pellets of HMM phi ranged between 2 and 100 micrograms/10(6) HMM phi for IgA and around 500 ng/10(6) HMM phi for IgM. Increased IgA release occurred when HMM phi were cultured in medium with a pH lower than 3. IgA concentrations were not very different between pH 5 and pH 7 and alkalinization of the medium did not produce any significant effect on IgA release. Release of IgA and IgM was not significantly increased when HMM phi were cultured in the presence of phorbol myristate acetate, formylated peptides, latex particles, lymphocyte-derived chemotactic factor or cholate. The kinetic study of IgA release by HMM phi, both with or without adding these stimuli to in vitro culture, indicates that IgA release depends on time and that these substances do not have any effect on it.