Background: Pharmaceutical products containing proteins cause problems in testing for endotoxin and pyrogens. Many proteins interfere with the LAL test and the proteins are immunogenic in rabbits. The monocytic cell line Mono Mac 6 is an alternative assay for detection of endotoxin and other pyrogens. Objective: To evaluate the use of the Mono Mac 6 assay for quantitative detection of endotoxin in proteins. Method: The quantitative detection of endotoxin in the three pharmaceutical products human albumin, γ-globulin and somatropin was evaluated. Results: For the three proteins the detection limit of the Mono Mac 6 assay was far below the threshold endotoxin limit described by the European Pharmacopoeia. Interference of two of the proteins with the Mono Mac 6 assay was observed, but the problems could be overcome either by dilution of the product or by comparison of the test with an endotoxin standard curve prepared in a solution of the respective pyrogen-free protein. Conclusion: The Mono Mac 6 assay is a reliable method for quantitative detection of endotoxin in proteins.