Several types of 37-mm-diam filters were exposed to a variety of temperature and humidity conditions to evaluate the effects of these conditions on filter weight stability. Temperatures ranged from 5 to 32/sup 0/C, and relative humidity ranged from less than 10 to 80%. The magnitude of weight change was determined for each filter type and test condition. A set of reference masses was weighed to establish the precision of the weighing technique. Results revealed that some filters were weight stable while other filters showed weight changes of 400 to 500 ..mu..g. The precision of the weighing technique based on the reference masses was estimated to be +-12 ..mu..g. Of the test conditions, desiccation had the least effect on filter weight. Based on these data, a procedure to minimize filter weight variation is recommended.