This paper describes a cross sectional study in which biological monitoring was used to assess exposure to methylene dianiline (MDA) in a selection of United Kingdom industries that manufacture or use MDA. Samples of urine were collected from 411 workers, representing 45 factories engaged in various activities. All urine samples were analysed for MDA and its acetyl metabolites and results are reported as total MDA. In this study, 91% of postshift urine samples and 88% of preshift samples had less than 50 nmol MDA/mmol creatinine. Some evidence was obtained which showed that when exposure to MDA was through inhalation (as solid material or contaminated dust), postshift urine samples had higher MDA concentrations than samples taken preshift the next day. When exposure was most likely to be through the dermal route, urine samples taken preshift next day tended to have higher MDA concentrations than urine samples collected immediately postshift on the day of exposure. Therefore a biological monitoring sampling strategy for MDA must take account of the route of entry into the body. If exposure is likely to be via inhalation, postshift samples should be collected and if exposure is likely via the skin, preshift samples next day are more appropriate. The results show that in most factories, regardless of the route of exposure, it is possible to keep urinary MDA concentrations below 50 nmol/mmol creatinine. In the absence of a health based or hygiene based standard, the use of a "yardstick" as a target to aim for, which has been derived from good working practice across the industry, may be a useful way of helping to control exposure.