A simplified procedure for the enzyme inhibition assay to measure okadaic acid and DTX-1 in mussels, based on the use of a commercially available enzyme preparation, is presented. The detection limit is 10 ng of toxin per g of digestive glands. Using Certified Reference Material (MUS-2), high accuracy and good precision is demonstrated for contamination levels higher than 32 ng g−1. Twenty samples can be processed in about 9 h by one operator, at the cost of US$ 10 per sample. Some possibilities for further enhancing the sensitivity and reducing the processing time are discussed and a monitoring example is presented. Copyright © 1999 John Wiley & Sons, Ltd.