Exposure information was evaluated for two large chemical manufacturing facilities and a research and development center in support of occupational health studies of employees assigned to these facilities. Methodology and rationale underlying the exposure categorization are provided, and descriptive exposure statistics are presented for a sample of 774 employees. Analysis of work patterns and exposure profiles revealed that 1) employee transfers among various production work areas did not follow a predictable pattern, 2) over 41% of the chemicals identified were present in multiple work areas, and 3) individuals exposed to one chemical of toxicologic interest were also likely to be exposed to other similarly toxic materials. The use of both work area and chemical-specific exposure measures is recommended, as each may be helpful in addressing etiologic questions regarding complex work environments.