Inhibition and enhancement effects on the Limulus Amebocyte Lysate (LAL) assay were studied in a series of endotoxin exposure samples from various occupational environments. Two types of analyses were performed, with various dilutions and with spiked samples. Results showed that inhibition and enhancement occurred in some samples, leading to respective underestimation and overestimation of the endotoxin concentration. When inhibition or enhancement is present, the result of this study shows that large differences in endotoxin concentration may be found, depending on the dilution at which the sample is analyzed. False conclusions about exposure levels and their potential health effects might be introduced. Results show that quality control procedures need to become standard practice when endotoxins are analyzed in occupational environments and development of a validated, generally accepted protocol should be encouraged.