The usual answer is that Russia abolished slavery in 1723. Technically speaking, there were no more slaves in Russia after this point. In reality, it meant they were merged into the class of serfs, whose lives were barely distinguishable from the formally enslaved anyway.
State measures to increase the numbers of people liable to direct taxation in the late seventeenth and early eighteenth centuries led to the addition of two groups to the peasant estate ... All slaves, including those who lived in their owners' households, were added to the poll tax census in 1723. By making slaves liable to taxation, the state extended its jurisdiction to them, thereby abolishing slavery in Russia. There was no longer any legal distinction between slaves and seigniorial peasants, and former slaves were merged with the seigniorial peasantry.
- Moon, David. The Russian Peasantry, 1600-1930: the World the Peasants Made. London: Longman, 1999.
Agricultural slaves who lived in their own houses were converted to serfs earlier, in 1679. Either way, as the last sentence states, this "abolition" results from the erasure of legal distinction between slavery and serfdom from the state's point of view. Practically speaking therefore, slavery essentially continued as serfdom until Alexander II's reforms emancipated the serfs.
Finally, as happened in the late Roman Empire, the status of peasant degraded into serfdom and become almost undistinguishable from slavery; in 1723 the two estates were amalgamated. Confusion persisted long thereafter, as can be seen from the fact that serfs were often sold as chattels, without land, in spite of official efforts to restrict the practice.
- Hosking, Geoffrey A. Russia and the Russians: a History. Harvard University Press, 2001.