A microbiological test based on one-plate, seeded with Bacillus subtilis BGA, is frequently used in Belgium as a screening method to monitor antibiotic residues in kidney tissue of slaughter animals. The procedure as described in the National Legislation recommends test agar pH 7.2 from Merck. Since this type of agar is no more commercially available, two kinds of agar medium are proposed on the market: niertest agar from Biotrading and Standard II nutrient agar from Merck. Niertest agar, which is actually used by a few laboratories, is a ready-to-use medium manufactured from test agar pH 7.2 from Merck. Its composition is identical to those of test agar pH 7.2. Standard II nutrient agar is the second alternative culture medium which is used by a higher number of laboratories. The equivalence of the standard II nutrient agar medium to both identical culture media (test agar pH 7.2 and niertest agar) was evaluated during a national collaborative trial. Results of this study as well as the experimental procedure are presented