The primary objective of this study was to investigate the effects of cobalt (Co(2 +)), palladium (Pd(2 +)), platinum (Pt(4 +)) and vanadium (V(2 +), V(3 +), V(4 +) and V(5 +)) on the ability of the neutrophil chemoattractants C5a and IL-8, as well as the pneumococcal toxin, pneumolysin, to activate human neutrophils in vitro. Neutrophil activation was determined according to the magnitude of the increase in cytosolic Ca(2 +) concentrations using a fura-2/AM-based, spectrofluorimetric procedure, as well as by a chemotaxis assay using modified Boyden chambers. In initial screening studies, in which the metals were used at a fixed concentration of 25 mu M, the Ca(2 +)-mobilizing interactions of C5a, IL-8, and pneumolysin were unaffected by exposure to Co(2 +), Pt(4 +) and V(2 + - 5 +). However, exposure of C5a, IL-8, and pneumolysin to Pd(2 +) resulted in either partial (IL-8) or complete (C5a and pneumolysin) loss of Ca(2 +) -mobilizing and chemotactic activities. In dose-response experiments, these effects of Pd(2 +) were detectable at a threshold concentration of 6.5 mu M. These observations demonstrate that exposure to Pd(2 +) may compromise innate host defenses, a previously unrecognized potential health threat of environmental and/or occupational exposure to a ubiquitous heavy metal.