Unisexual females, (80-100 days post-infection) were cultured in vitro and exposed to mature male worms from either mixed or single-sex infections. A comparison was made of the uptake of [3H]thymidine (used as a marker for DNA synthesis) by isolated unisexual females or by those females exposed to males derived from either mixed or unisexual infections during in vitro culture. All males were observed to be paired with females by approximately 18 h post-culture. At 24 h post-culture females paired with males from mixed infections had an elevated uptake of [3H]thymidine compared to females that had not paired and uptake was also significantly different (P less than or equal to 0.001) from females exposed to males from unisexual infections. By 48 h there was no significant difference between uptake in females paired with either type of male. The frequency of uptake in the paired females was approximately double that in unstimulated females while non-paired females in the presence of males had an increase of between 35 and 45%. In a second experiment the effect of depriving a mature egg-laying female of the paired male was investigated. These females had a continual decrease in uptake of [3H]thymidine compared to mature females maintained with male partners. The uptake of [3H]thymidine was used as a marker for DNA synthesis and the effect of the male worm on DNA synthesis in the unisexual female is discussed in view of the above results.