Probit expressions are widely used in quantitative risk analysis to predict the human response to inhalation of toxic gases. This paper presents a comparison of predicted lethal concentrations using two authoritative sources for the probit equation constants, i.e. publications of the Center for Chemical Process Safety of the American institute of Chemical Engineers and of the Netherlands Organization of Applied Scientific Research. The comparison shows that the predicted lethal concentrations are significantly different for a number of important chemicals. The basis of the probit constants for ammonia is examined in detail. Some suggestions are made regarding further work in this area.