We studied the respiration and glucose utilization of suspensions of cultured lymphocytes. Respiration was measured by a couloximetric method using closed systems without a gas-suspension interface. The sensitivity of the method allowed measurements during a time interval of less than 1 hr. We found that the respiration of lymphocytes was strongly density dependent over the range of 10(8)-10(3) cells/ml. Oxygen utilization per cell increased 100-fold with 1000-fold dilution of the cell suspension. Glucose utilization showed little density dependence. We showed that the phenomenon was not due to nutrient depletion, pH shifts, accumulation of lactic acid, nor damage of cells during the dilution. Since in our experiments a gas-liquid interface was absent, the previously suggested explanation of gas diffusion as a limiting factor has been excluded. The experiment was best explained by regulation of respiration by humoral factors produced by the cell suspension.