The performance of commercially available portable instrumentation for detecting nitrogen dioxide, sulfur dioxide, and hydrogen sulfide was evaluated. Length-of-stain detector tubes from several manufacturers were included in the study. The instruments were evaluated in atmospheres with 50- and 80-percent relative humidities at 75 deg f and containing test gas concentrations of 0.1, 0.25, 0.50., 1.0, 1.5, and 2.0 Times the threshold limit values. The results from the gas analyzers were correlated with the known contaminant concentration by calculating and plotting an estimated line of regression, confidence limits, and correlation coefficient for each detector system. An increase in the relative humidity from 50 to 80 percent at 75 deg f changed the instrument response to the contaminants. The uncertainties in detection precision reported are for the observer-instrument system. The main difference or inconsistency in toxic gas concentrations determined with length-of- stain tubes resulted from the observer's interpretation of the results rather than from a predominant difference in the tubes.