AbstractProteus vulgarisis shown to take up nicotinic acid in the early stage of growth in amounts greater than needed for growth. The time variation of the specific nicotinic acid content of the cells, calculated by dividing the amount of nicotinic acid taken up from the liquid medium by the mass of cells present at that time, is interpreted to define two parameters, the maximum specific nicotinic acid content,f1, and the minimum content required for growth,f2The difference,E, between these parameters is the specific nicotinic acid content capable of supporting growth for three doublings after depletion of nicotinic acid from the medium. A kinetic model for the system is developed based upon two stages of growth, a stage in which the cells accumulate from the medium their maximum nicotinic acid content, and a stage in the nicotinic acid‐depleted medium in which cell growth occurs at the expense of the cell‐bound nicotinic a