Neutron activation analysis of a large number of samples of midwestern cherts shows systematic variation of trace-element proportions within several chert formations. This chemical variation parallels variation in physical properties of the cherts and can also be found between different outcrops of the same chert. Surface alterations of artifacts after they have been deposited in soils also appear to change the chemical properties of the materials to some degree. All of these factors will complicate attempts to characterize chert sources and assign artifacts to them on the basis of their trace-element compositions. Nevertheless, adequate sampling and analysis of cherts should allow discrimination of sources in most areas, sincethe patterning of variation between cherts appears to be meaningful. -- AATA