Calcium influx was studied in monolayers of HeLa cells to determine the number of exchangeable and nonexchangeable pools and the rate constant of the different fluxes. Of the two exchangeable pools, one has a very fast rate of exchange with a half-time of 1.54 min, a compartment size of 1.06 mmicromoles/mg cell protein, and an exchange rate of 474 micromicromoles/(mg protein\.min). This compartment is likely to be extracellular and could represent calcium exchange between the extracellular fluids and surface binding sites of the cell membrane. The second exchangeable pool has a half-time of exchange of 31 min, a compartment size of 2.69 mmicromoles/mg cell protein (0.224 millimole calcium/kg cell water), and a flux rate of 0.0546 micromicromole cm(-2) sec(-1). This compartment can be considered to be the intracellular pool of exchangeable calcium. An unexchangeable intracellular pool of calcium of 3.05 mmicromoles/mg cell protein was detected implying that only 45% of the intracellular calcium is exchangeable. In addition, a large extracellular pool of calcium has been found to be unexchangeable, probably a part of the cell glycocalix. Finally, dinitrophenol 10(-3)M does not affect the slow component of the calcium uptake curve which brings new evidence that calcium entry into the cell is not a metabolically dependent process.