W e have performed a comprehensive study of calcium tracer flux, distribution, content, and ionized cytoplasmic calcium during monocyte activation. A model of monocyte calcium was developed from 45Ca uptake and exodus curves which indicated that cell calcium was partitioned between three compartments. The magnitude of the time constants for each pool lead us to propose cellular locations for these three compartments: a surface plasma membrane pool, a cytoplasmic pool, and an organelle pool. 45Ca uptake and exodus experiments were analyzed using a nonlinear least squares fit of compartmental exchange rates and sizes. The production of superoxide was used as a reflection of the state of activation of the monocytes treated with Concanavalin A (Con A). We found that Con A-treated monocytes have an increase in the calcium exchange rate with the cytoplasmic pool from 0.04 to 0.07/min (P less than 0.05), and an increase in the size of the cytoplasmic pool from 0.08 to 0.13 pmol/cell (P less than 0.05). There were no significant changes in the exchange rates or sizes associated with either of the other two compartments. The cytoplasmic ionized calcium was measured with the fluorescent probe, Quin 2, which indicated a resting level of 83 nM free calcium in unadhered monocytes. Con A stimulation caused a doubling of the cytoplasmic free calcium to 163 nM within 45 s. This increment in cytoplasmic free calcium preceded the onset of superoxide following Con A treatment. These studies indicate that Con A binding to the plasma membrane increases the monocyte plasma membrane permeability to calcium. External calcium enters the cell at an increased rate and contributes to both internally bound and free calcium. The magnitude of the increase in free calcium is proportional to the concentration of Con A and stimulates calcium extrusion via the calcium transport ATPase. Moreover, there is an increased concentration of ionized cytoplasmic calcium which has the potential to interact with other cellular regulators that modulate cell activation and superoxide production.