The results of an evaluation of substitute organic vapor cartridge test agents to replace the possibly carcinogenic carbon- tetrachloride (56235) were reported. Based on physical characteristics, toxicity, and reported charcoal absorption characteristics, four possible replacement agents, ethyl-acetate (141786), pentane (109660), n-hexane (110543), and heptane (142825), were evaluated. These agents were tested on respirator cartridges and compared with results obtained using carbon-tetrachloride. Breakthrough data for 1,000 parts per million (ppm) ethyl-acetate were similar to the baseline 1,000ppm carbon-tetrachloride data; however, its water solubility precluded its usefulness. The pentane breakthrough time at 500ppm was the closest to the carbon- tetrachloride baseline curve, while 1,250ppm heptane were needed to give breakthrough times of the same order as those seen with 1,000ppm carbon-tetrachloride. n-Hexane gave 1,000ppm data comparable to the 1,000ppm carbon-tetrachloride data without interference from other variables. Side by side testing of the two best replacement candidates, pentane and n-hexane, with carbon- tetrachloride indicated that 550ppm pentane and 1,000ppm n-hexane would be suitable replacement agents for 1,000ppm carbon- tetrachloride in evaluations of respirator breakthrough times. The authors conclude that due to the lower toxicity of pentane compared with n-hexane, pentane appears to be the test agent of choice to replace carbon-tetrachloride.