Most modern Eastern Orthodox Christians claim that they do not accept the Catholic understanding of the Immaculate Conception, which can be briefly summarized as the teaching that the Holy Theotokos was preserved free from all stain of original sin at her conception by the merits of Christ our God, applied retroactively (back in time) from His salvific and redemptive passion and death.
Here I set aside the question of whether original sin is even a legitimate doctrine (which could easily take up a whole series in itself) and focus on the claims that the Theotokos did have original sin but was either purified from it at some later point in her life, or always had original sin but never committed actual sin.
To the contrary, it seems to me the ancient Byzantine tradition clearly teaches the Theotokos was preserved from all traces of sin, whether original or actual, in a way that while not accidentally identical to Latin formulations, still affirms the same essence as the dogma solemnly defined by Pius IX, Pope of Rome, in 1854.
The first source I would consider is the work of liturgist Fr. Casimir Kucharek. In his text The Byzantine-Slav Liturgy of St. John Chrysostom, Fr. Kucharek notes that Byzantine Christians, both Catholic and Orthodox, call upon the Theotokos as “the immaculate, spotless one” eight times just during the Divine Liturgy. But one could object this implies nothing about her conception being immaculate.
However, Fr. Kucharek further observes that her immaculateness is emphasized especially on the feast day of her conception in the Byzantine Church, pointing to the Office of Matins: “This day…begins to take being the spotless lamb, the most pure tabernacle, Mary,” “she is conceived…the only immaculate one.” 
This is the ancient Byzantine tradition. Even when medieval Latin theologians such as St. Thomas Aquinas notoriously questioned this belief, their Byzantine counterparts confessed it unanimously! It was only later, in the fifteenth century for the Greeks, and not until the seventeenth for the Slavs, that the belief began to be seriously challenged, and not without great resistance by the faithful. 
In Russia many pious Orthodox rejected the Greek novelty as blasphemous, especially the Old Ritualists who held fast to the ancient customs and beliefs they had received. Orthodox opposition to the Immaculate Conception only solidified in very recent times after Rome’s definition in 1854. Yet even today the Orthodox Church has never made any definitive pronouncement against it. 
This fact confirms my view that as in many other disputes, Orthodox objections to Catholicism have much more to do with ecclesiastical-political controversies than genuinely insurmountable theological, doctrinal, and dogmatic differences.
 Casimir Kucharek, The Byzantine-Slav Liturgy of St. John Chrysostom, (Alleluia Press: Allendale, NJ, 1971), 355.
 Kucharek, Liturgy, 356.
 Kucharek, Liturgy, 357.