A system for measuring oxygen consumption uses a chamber holding culture medium with a biological material. A resistively gas-permeable membrane has a first side contacting culture medium in the chamber and a first sensor for measuring oxygen in the culture medium. A second sensor measures oxygen in gas adjacent a second side of the membrane. The sensors provide data to a processor having a memory with machine readable instructions for determining oxygen consumption from the data. Alternatively, a method of measuring oxygen flux begins with placing biological material, with fluid, in the chamber; exposing the fluid to a resistively-permeable membrane with oxygen-containing gas on a second side of the membrane; measuring oxygen in the fluid; measuring oxygen in the gas, and calculating the oxygen flux of the biological material from an oxygen permeability constant of the membrane and a difference between the oxygen concentrations of the fluid and gas.