Osteoclasts and activated macrophages in culture were shown to generate an acidic microenvironment specifically in the attachment zone between the cell and the base of the culture dish. Measurements using pH microelectrodes revealed that osteoclasts, when firmly attached, could achieve a pH fall of about 1 unit min-1 to a limit value of pH 3.0 or less. Activated macrophages produced a slower fall of 0.5-2 pH units h-1 and a limit value of pH 3.6-3.7 was generally detected. The method of activation was relatively unimportant, but where macrophages formed clumps the pH effect was reinforced. Osteoclasts were also examined in situ in osteoporotic bone fragments in rabbit ear chambers, using a combination H+ and Ca2+ double-barrelled electrode. The pH readings reached a lower limit of 4.7 and the calcium concentration rose to a maximum of 40 mM in the erosion sites. In vivo such acid conditions would favor the direct extracellular action of secreted lysosomal proteinases in the degradation of collagen by both cell types.