Passionate kissing swaps more than just spit, with DNA lingering in a partner's mouth for at least an hour.
Researcher Natalia Kamodyova and her colleagues at Comenius University in Bratislava, Slovakia, found 12 couples willing to make out for at least two minutes in the name of science.
Saliva samples were then taken from each of the women at five, 10, 30 and 60 minute intervals.
The researchers found DNA from the men could still be identified in the samples up to an hour after the couples had kissed.
The study, entitled "Prevalence and persistence of male DNA identified in mixed saliva samples after intense kissing", has been published in the journal Forensic Science International: Genetics.
The method used by the researchers relied on the detection of the Y chromosome, so it could only identify male DNA.
The findings could prove vital in cases of sexual assault, or could perhaps be used to catch out an unfaithful partner.
"Our study confirms that foreign DNA tends to persist for a restricted period of time in the victim's mouth, can be isolated from saliva after prompt collection and can be used as a valuable source of evidence," the researchers wrote.