Evidence linking soluble aggregation intermediates of the amyloid-beta protein (A beta), as well as the ongoing growth of A beta aggregates, to physiological responses characteristic of Alzheimer's disease (AD) indicates that a kinetic description A beta aggregation intermediate growth may be fundamental to understanding disease progression. Although the growth of mature A beta fibrils has been investigated using several experimental platforms, the growth of A beta aggregation intermediates has been less thoroughly explored. In the current study, a quartz crystal microbalance (QCM) was employed to analyze the real-time growth of A beta(1-40) aggregation intermediates selectively immobilized on the crystal surface. Immobilization permitted quantitative evaluation of A beta(1-40) aggregation intermediate growth under controlled solution conditions. Elongation of A beta(1-40) aggregation intermediates via monomer addition proceeded in a nonsaturable and reversible fashion. The rate of elongation was observed to vary linearly with both monomer concentration and immobilized aggregate density, to be elevated by increases in solution ionic strength, and to increase as solution pH became more acidic. Elongation was consistent with a first-order kinetic model for the single growth phase observed. These findings extend previous kinetic studies involving the growth of mature A beta fibrils to describe the growth of A beta(1-40) aggregation intermediates via monomer addition.