Background: Derived No Effect Level (DNELinh) has been set for occupational exposure to but-2-yno-1,4-diol according to REACH principles. Maximum allowable concentration (MAC) and DNELinh have been compared. Material and Methods: Experimental data from two inhalation studies on rats and three oral studies have been used to calculate DNEL. Results: Estimated DNELinh values show significant differences and fall within the range of 0.33-0.02 mg/m³, depending on the chosen experiment and critical effect. It seems that a 30-day inhalation study best reflects the penetration of xenobiotic to the human organism. This experiment has been used to set MAC value of but-2-yno-1,4-diol of 0,25 mg/m³ and it is close to local DNELinh - 0.10 mg/m³. Both values have been estimated on the same starting point NOAECloc - 0.5 mg/m³, but different assessment factors have been applied. Conclusions: A general feeling is that because of the differences in methodologies for calculating DNELs versus those used for calculating health-based OELs, the DNEL will tend to be lower than any corresponding health-based OEL for that chemical. This indicates that the OEL does not provide the appropriate level of protection required by REACH. The calculation leads to a new value (DNEL) that requires different risk management measures and operational conditions. Med Pr 2009;60(5):347-357