The dissolution of aerosol 57CO3O4 particles was investigated by exposing baboon alveolar macrophages (AM) to the particles and immobilizing the AM in alginate macrobeads crosslinked with Ca2+ ions. AM were obtained from bronchopulmonary lavage and, after immobilization, were stored in culture medium enriched with Ca2+. Dissolution of 57Co3O4 particles by these AM was measured for up to 16 days by gamma counting of filtered media containing beads. For these particles, the daily dissolution rate was 0.242 per cent versus 0.072, 0.089 and 0.091 per cent for beads containing particles only, particles combined with AM enzymes, and non-phagocytic cells, respectively. The rate obtained with immobilized AM is very close to the rates previously found for adherent AM in culture. Moreover, AM that had phagocytosed PuO2 particles before immobilization by the same method gave a mean daily dissolution rate close to that obtained in vivo with baboons. We also demonstrated, by dissolving the alginate network, that immobilization did not affect AM viability. The present method of immobilization seems very promising compared to conventional methods because it facilitates manipulation, allows the use of cells at a high density in a small volume, and gives results for particles with a very low dissolution rate.