Problems with urinary monitoring as the primary method of assessing 4,4'-methylenebis(2-chloroaniline) (101144) (MBOCA), an animal carcinogen, in the workplace are discussed. No completely satisfactory method exists for monitoring the workplace environment for MBOCA, since air sampling and wipe sampling are inadequate. Thin layer chromatography, gas chromatography electron capture (GCEC) and high performance liquid chromatography techniques in use were compared. One problem with guidelines for interpreting sample results is that only animal data are available for extrapolation to absorbed dose in humans and metabolism of MBOCA in humans may be quite different from its metabolism in rats and dogs. In planning the frequency with which urine samples should be collected, the kinetics of excretion and whether the exposure is continuous or intermittent must be considered, since these factors influence the degree of fluctuation in the measurement over a period of time. A potential problem is that many companies are sampling at monthly or quarterly intervals, which may be too infrequent to reflect changes in individual exposure or in workplace conditions. Variation in MBOCA concentration of the urine among workers or in a given worker over a period of time can be controlled by reporting urinary MBOCA concentrations as microgram per gram of creatinine, rather than microgram per liter. Ambiguity about how to interpret the results of urinary monitoring for MBOCA complicates the use of data for regulatory purposes and for medical removal. Genetic variation may influence urinary concentrations among individuals. Biological monitoring data can be a valuable epidemiological tool if the results are systematically recorded and analyzed. The authors suggest that caution should be used against over reliance on present monitoring programs.