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Epididymal sperm were collected from C57Bl6/J X DBA2/J (B6D2) males and allowed to capacitate for 2 hr. When cumulus-free oocytes were exposed to sperm for 15 min in either the presence (6.0 mM) or absence of caffeine, fertilization did not occur. However, when cumulus cells were left intact, 23% of oocytes were fertilized in caffeine-free medium and 62% in caffeine-containing medium. When cumulus-free oocytes were incubated with sperm for 30 min, none was fertilized in the absence of caffeine, but 33% were fertilized when 6.0 mM caffeine was present (P less than .02). These effects of caffeine were on the sperm, as sperm exposed to caffeine and then coincubated with oocytes for 15 min in essentially caffeine-free media fertilized a similar percent of oocytes (93%) as when sperm and oocytes were exposed to caffeine during the fertilization period (86%). When sperm were capacitated in caffeine-containing medium, the percentage of ova fertilized was similar to capacitation without caffeine. We conclude that both cumulus cells and caffeine speed up the fertilization process with mouse gametes and that the effect of caffeine is on the sperm, but not due to more rapid capacitation.