A NIOSH sponsored effort to develop and test a prototype service life indicator for organic vapor air purifying respirators is reported. A series of performance requirements for an end of service life indicator (ESLI) was developed which addressed indicator characteristics, specificity, reliability, respirator fit, weight, impairment of vision, durability, breathing resistance, temperature and humidity, electrical components, and toxic materials. A theoretical evaluation of various detection approaches was conducted to select a specific developmental approach. It was found that the metal oxide gas sensor (MOGS) merited further assessment because of its low cost, commercial availability and desirable nonspecificity. Also, catalytic oxidation was studied. Experimental evaluation showed that the MOGS responded to a large number of organic vapors within a relatively small concentration range. A prototype ESLI incorporating the MOGS was designed and constructed. The device consisted of a belt or shirt pocket mounted battery pack connected to a detector/indicator unit fitted between the respirator facepiece and the chemical cartridge. It is recommended that the ESLI be tested under actual use conditions, and that the MOGS detector be tested against additional challenge atmospheres.