Urinary asbestos concentrations were evaluated as an indicator of occupational exposure to chrysotile asbestos via inhalation and ingestion. Detection of asbestos in the urine represents the first step in developing a biological indicator of exposure. Such an indicator could be used to supplement exposure data from workplace air sampling. A biological indicator would be particularly valuable in evaluating workers with intermittent airborne asbestos exposures and in determining if airborne exposure results in penetration through the lung or gastrointestinal tract. Transmission electron microscopy was selected as the most sensitive technique for identification of all sizes of asbestos fibers which might appear in the urine. First morning void urine samples were obtained from six workers (occupationally exposed to chrysotile asbestos in a factory producing roof coatings) and from a control group (six individuals with no occupational exposure). The levels of chrysotile asbestos detected in the urine of five workers were significantly greater than the asbestos concentrations in matched field blanks (both on a number and mass basis). Field blanks were designed to detect asbestos in the urine samples due to contamination which might occur during urine collection. Also, the workers' urinary asbestos levels were significantly greater than the concentrations found in the control group (both on a number and mass basis). Finally, the levels of chrysotile asbestos detected in the urine of two of six controls were significantly greater than those in matched field blanks (both on a number and mass basis). Although the project was not specifically designed to correlate urinary and airborne asbestos concentrations, preliminary data indicated that a correlation did not exist between these factors.