Those opposing the ordination of women deny any historical precedent. However, the presence of women in the priestly ministry of the early church has been ignored or denied. Giorgio Otranto, director of the Institute of Classical and Christian Studies, University of Bari, Italy believes evidence of women priests is found in an epistle of Pope Gelasius I (late 5th c). His epistle was sent to bishops in three regions in southern Italy. One of his decrees in this epistle states,
"Nevertheless we have heard to our annoyance that divine affairs have come to such a low state that women are encouraged to officiate at the sacred altars, and to take part in all matters imputed to the offices of the male sex, to which they do not belong."
This Pope condemns very harshly the conduct of bishops who went against certain church canons by conferring priestly ordination on some women. He is probably referring to canons from four councils which took place within a 100 year span starting in the second half of the 4th century; the councils of Nicaea, Laodicea, Nimes and the first council of Orange (441). These church councils prohibited women from participating in the liturgical service in any way, or from being members of the clergy.
Professor Otranto thinks these prohibitions prove just the opposite. "If the church councils banned the ordination of women as priests or deacons that must imply that they really were ordained." Otherwise, why ban them? As Otranto says, "A law is only created to prohibit a practice if that practice is actually taking place - if only in a few communities."
He points to the presence of women priests (presbyterae) in the area of Tropea, in Calabria where there is an inscription from a sepulchre referring to Leta presbytera. It is dated 40 years before Gelasius’ letter, a date and location that indicate she probably was one of the women to whom Gelasius was referring. In the term ‘presbytera’ one should see, Otranto believes, "a true and proper female priest, and not the wife of a male priest, as other scholars have held on the impulse of a Catholic historiographic tradition that has never made any concession to the female priesthood."
Another presbytera is recorded in an inscription on a sarcophagus in Dalmatia and bears the date of 425. The inscription reads that a plot in the cemetery of Salona was purchased from the presbytera Flavia Vitalia. Here a presbytera (female priest) has been invested with an official duty, which from a certain period on was appropriate to a presbyter.
So far fifteen archeological inscriptions have been found that indicate ordained women. Rome maintains these women were ordained by heretical groups.
http://www.womensordination.org/why.html