The results of a qualitative fitting test for half-mask respirators utilizing a sodium saccharin aerosol were compared to the results of a quantitative fitting test utilizing an oil mist aerosol. For a respirator-wearer combination having a fit factor < 10, as determined by the quantitative test, the point estimate of the probability of that combination not being rejected by the saccharin qualitative test was 0.00 with 95% confidence that this statistic was not expected to exceed 0.05. For a respirator-wearer combination having a fit factor < 100, the point estimate of the probability of the combination not being rejected by the qualitative test was 0.03 with 95% confidence that this statistic was not expected to exceed 0.08. The effects of temperature and relative humidity on the size distribution of the saccharin aerosol were also investigated and found to be slight.