Phagocytosis of alveolar macrophages (Mphis) toward poly(lactic-co-glycolic acid) (PLGA) microspheres (MS) loaded with the anti-tuberculosis agent rifampicin (RFP-PLGA MS) has been shown to be effective for the treatment of tuberculosis. The phagocytosis should be evaluated in terms of that toward reference MS. We chose polystyrene latex (PSL) MS as a reference. In this study, phagocytic activity of cell line NR8383, derived from rat alveolar Mphi, toward PSL MS with various diameters was examined by incubating the cells for 4h at 37 degrees C with various numbers of PSL MS per Mphi cell (MS/Mphi=0.1-10). The results were then compared with those of the phagocytosis toward RFP-PLGA MS. We determined the phagocytic activity by counting the population of Mphi cells that had phagocytosed MS (N) and the number of particles phagocytosed (n) in microscopic fields. Both N and n for PSL and RFP-PLGA MS increased in general with an increase in MS/Mphi, but both of these values for PSL MS were smaller than those for RFP-PLGA MS. Phagocytosis of the particles were dependent on the particle size; i.e., of the PSL MS the 6-mum ones were taken up by Mphi the most, and the RFP-PLGA MS 3 microm in diameter seemed to be phagocytosed the most efficiently, although we were not able to determine exactly the phagocytosis of 6- and 10-microm RFP-PLGA MS. From the changes in N and n values with MS/Mphi, the phagocytosis of RFP-PLGA MS was likely to enhance the phagocytic activity of Mphi cells, but this effect did not seem to be significant for PSL MS.