Uptake kinetics of cadmium and zinc in gram-positive bacteria, Bacillus firmus, isolated from Hong Kong sediments were examined in the present study. The metal uptake by the bacteria was measured at different ambient free metal ion concentrations (10(-12)-10(-6) M Cd(2+) and 10(-10)-10(-6) M Zn(2+)) by adding different concentrations of total dissolved Cd and Zn and hydrophilic organic ligands (ethylenedinitrilotetraacetic acid, nitrilotriacetic acid, and citrate). Our data suggest that Cd and Zn uptake by B. firmus is best predicted by Cd(2+) and Zn(2+) activities. Free metal ions were complexed with the active sites on the bacterial surface, and an equilibrium between the free metal ion and surface-metal complex was reached quickly. After binding, the metal ions were then biologically transported into the bacteria. In addition, with the presence of lipophilic organic ligands (diethyldithiocarbamate and oxine), the lipophilic metal complex was internalized rapidly into B. firmus by passive diffusion through the bacterial plasma membrane. The uptake of the lipophilic metal complex could not be predicted by the free ion activity model because the mass transport through plasma membrane was the most important metal uptake pathway. Furthermore, the efflux of Cd and Zn by B. firmus was determined in the present study. The calculated efflux rate constants of Cd and Zn were (5.55 +/- 1.96) x 10(-4)/min and (3.75 +/- 1.04) x 10(-4)/min, respectively. The present study helps us to understand the process of bioaccumulation of metals in marine bacteria, which remains a poorly studied area.