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The prospect of monitoring in-service particulate emissions of transit buses by examining the cumulative records of engine-lubricating-oil solids was investigated. This possibility was suggested by the observation that the same particulates emitted from engine exhaust also enter the engine crankcase. To test this hypothesis, four in-service metropolitan transit buses were selected for their varying exhaust-smoke levels. The relationship between lubricating-oil solids to exhaust opacity levels was investigated by evaluating the results of some periodic oil-analysis and smoke-opacity tests. This evaluation is presented along with the procedures used to obtain the data. The specifications and in-service operating profiles of the buses are also described. The evaluation revealed a sufficiently strong correlation between opacity and total suspended solids in the engine lubricating oil to warrant further development with a view to practical applications.