A jar-rolling apparatus was constructed to prepare test substrates using sediments spiked with laboratory chemicals, the toxicity of which were assessed with the Rhepoxynius abronius bioassay. Test sediments were mixed by rolling them for several hours in one-gallon glass jars. Mixing was considered adequate based on the analytical results of sediment samples that were collected at locations along the longitudinal axis within each horizontally lying jar immediately after rolling. However, because comparative standards did not exist, definitive conclusions of mixing thoroughness could not be drawn. Coefficients of variation, used to assess mixing within jars, were 11.5% or less (mean 5.1%). Mixing was not significantly different among replicated jars but, in some jars, was significantly different among within-jar sample locations.