This in vitro study examined the effect of eluate from heat-activated, chemically activated, and microwave-activated denture base resins on cell viability of primary cultures of human gingival fibroblasts. Eluates corresponding to 24, 48, 72, and 96 hours of resin disk immersion were prepared. Fibroblasts were plated at a density of 3 x 10(4) cells in 96-well plates and exposed to a medium containing eluate. After 24 hours, the cytotoxic effect was determined by cellular mitochondrial function. The effect of eluates was compared to control cultures containing culture medium without eluate. Results indicated that at all time periods tested, all three resins leached materials that were cytotoxic to the fibroblasts. Eluate from chemically activated resin disks was more cytotoxic than eluate from heat-activated and microwave-activated disks. In general, cytotoxicity appeared to diminish as disk immersion time was increased. The greatest cytotoxic effect on cell viability was observed with eluates recovered after 24 hours of disk immersion, and the least cytotoxic effect was observed with eluates recovered after 96 hours of immersion.