ETX (EcoToX) is a computer program for MS-DOS computers for the extrapolation of laboratory toxicity data to environmental concentrations that are safe for the majority of the biological species. It is based on the theory of Van Straalen and Denneman (1989), as modified by Aldenberg and Slob (1993). The estimates of the so-called Hazardous Concentrations can be given on two levels of confidence: a median estimate that in the long run will equally often overestimate, as well as underestimate, the true HC5 (95% species protection), and a 95% confidence underestimate that is almost surely on the safe side. Data can be entered interactively in ETX and saved to files. One can also compose data files with a text editor and read them in with ETX. Next to the computation of Hazardous Concentrations, one can calculate the hazard to the biological community at given levels of exposure, i.e. environmental concentrations. All results can be saved to files, for later printout, or study.