This paper deals with the effect of calcium binding in the cell wall on the measured 45Ca influx in Chara corallina Klein ex Will. esk. R.D. Wood. Calcium in the cell wall was in the range 687-1197 gmol-m -2 compared to the sap which contained only 144-256 gmol 9 m -2. In dilute culture solutions the calcium content of the cell wall was relatively independent of external calcium at concentrations above about 0.1 mol. m -3. The half-times for exchange of calcium from 45Ca-labelled cell walls varied from 45 min at 0.05 mol. m -3 to less than 2 min at 2 mol-m -3. The effectiveness of other cations in displacing calcium from cell walls was in the order La > Zn > Co > Ni > Mg. Rinsing of 45Ca-labelled cell walls in 2 mol 9 m -3 LaC13 for 20min removed more than 99% of the bound 4SCa. However, the residual 4SCa activity in isolated cell walls following La 3 ยง rinsing was similar to that in whole cells. It is concluded that in whole cells 45Ca influx cannot normally be distinguished from extracellular binding of calcium. Methods are described for the measurement of 45Ca fluxes in charophyte cells by isolation of intracellular 45Ca after the uptake period using techniques which avoid contamination from the large amount of tracer bound in the cell wall. At an external calcium concentration of 1 mol 9 m-3, the plasmalemma influx was approx. 0.2 nmol 9 m-2. s-1 of which about half entered the vacuole and half was effiuxed back into the external solution. The cytoplasm filled with calcium with a half-time of 40-50min with an 'apparent' pool size of 50 mmol-m -3. After 2 h the net flux to the cell was almost the same as the vacuolar flux. The fluxes reported are an order of magnitude lower than previously reported calcium fluxes in plants.