Diffusive sampler operation may be affected by a large number of factors. Interactions between the factors may also be important. A protocol for determining sampler performance has been developed by the National Institute for Occupational Safety and Health (NIOSH) that addresses sampler precision and accuracy when these factors vary within the ranges normally encountered in the field. However, it is costly, time-consuming, and often unnecessary to apply all of the tests to each sampler for every possible chemical hazard. A sampler shown to sample a specific vapor accurately may be assumed to require fewer tests to assure equal performance with related chemicals. The criteria used to define the reduced tests depends somewhat on sampler design, and the justification for applying the criteria must depend on information gained from validating a large selection of related chemicals using the full protocol. Thus, the full protocol remains the standard of reference for the lower validation level. Based on these considerations a "bilevel validation" approach was developed for the SKC 575 series diffusive sampler. Application of the bilevel approach to the NIOSH protocol is described, and the results are related to the full NIOSH protocol and also to a European validation protocol. The European protocol recognizes the validity of reduced tests for homologue validation. The conclusions are that (1) it is feasible to validate a sampler for a large number of organic vapors using the NIOSH protocol and that this number can be substantially increased by use of a bilevel approach, saving time and resources, and (2) samplers validated to either the NIOSH or European protocol should be accepted as meeting the requirements of the other.