Work will begin in Italy next month on a monumental wall sculpture for the Basilica of the National Shrine of the Immaculate Conception in Northeast Washington. Installation of the 50-ton marble artwork, planned for fall 1998, will complete major structural additions to "America's Catholic Church."

"It's a big piece of work, let me tell you," said project architect Anthony J. Segreti, who designed an aluminum shelf for the sculpture to rest on and a steel support system to reinforce the wall.

The sculpture, titled "The Universal Call to Holiness," will be 52 feet long and 15 feet wide. It will hang 19 feet above the floor at the rear of the nave, said Segreti, who was supervising architect at Washington National Cathedral -- the shrine's older and slightly larger Anglican counterpart across town -- during its completion in 1990 and who remains its consulting architect.

The work was conceived by Cardinal James A. Hickey of the Archdiocese of Washington and designed by Silver Spring artist George Carr, who calls the sculpture a "high relief," meaning the figures are cut more deeply into marble for more detail than those in a bas relief.

Carr made drawings and created scale models in a rented studio near the shrine in Brookland. He then commuted four days a week for seven months to a studio in Chester, Pa., that was selected to construct a full-scale model of Styrofoam and plasteline, an artificial oil-based clay. Carr's task was to carve details of figures in the model, which was cast in plaster for shipping to Europe.

"It's the hardest stint of work I've ever done," said Carr, who created a monumental sculpture last year for Brigham and Women's Hospital in Boston and assisted Frederick E. Hart on statues of former president Jimmy Carter and Richard B. Russell, the Democrat from Georgia who was one of the all-time parliamentary masters in the Senate. Carr will make trips to Italy to observe the translation of his design into slabs of Botticino marble more than a foot thick.

Segreti said he isn't quite sure how he will get the marble sections through the basilica doors. Most likely, a crane will lift the 16 pieces to the top of the entrance stairs, where they will be transported through the small openings by "some large, rubber-tired vehicle."

Building scaffolding to raise the marble into place will take about six months, and the carving up to eight months at a studio in Pietrasanta, on the west coast of Italy. Once the marble reaches the basilica floor, installation will take about three weeks.

The project also includes placing marble cladding over the masonry walls of the narthex, just behind the sculpture, said Monsignor Michael J. Bransfield, rector of the church for the last 11 years. The entire interior of the Great Upper Church (the nave) was finished with brown brick when the shrine was dedicated in 1959. Marble cladding was laid over the brick as money allowed, beginning around the time of Luci Baines Johnson's wedding at the shrine in 1966.

The cost for the project could reach $2 million, Bransfield said. Joseph V. and Bertha Braddock, of Alexandria, contributed $1 million through their charitable organization, the Aztec Foundation. Other contributors will make up the balance, basilica spokesman Kevin Mukri said.

The shrine, begun in 1920 and declared a basilica in 1990 because of its historical importance and significance as a worship center, has been the scene of ongoing interior construction for 40 years. Most of the work in the Byzantine-Romanesque building has involved construction of 72 chapels or oratories (for prayer but not Mass) around the perimeter of the 460-foot main church and around the smaller crypt church below.

The most recent chapels are the Filipino Oratory, dedicated in June, and the Chapel of Our Lady of Hope of Pontmain, France, given in 1994 by Bob and Delores Hope in memory of the comedian's mother. The last scheduled chapels will be dedicated in the crypt area this month: the Indian community's Our Lady of Valankanni Chapel, on Aug. 16, and the Mother of Africa Chapel, on Aug. 30.

The Hope chapel cost $1 million; the three other chapels cost about $400,000 each.

The Rev. Johann G. Roten, director of the International Marian Research Institute at the University of Dayton, said that the basilica likely has the most diverse offering of ethnically supported chapels of any church in the world.

Holy sites such as Lourdes, France, where the Virgin Mary is said to have appeared to a young girl in 1858, might attract a wider range of nationalities among its millions of pilgrims each year, Roten said. But he could think of no church architecturally configured with as many chapels dedicated to Madonna figures from around the world, among them: Our Lady of Czestochowa (Polish), Our Lady of Lujan (Argentine), Our Lady of Brezje (Slovenian), Our Lady of Siluva (Lithuanian) and Our Lady of Guadalupe (Mexican).

Although these chapels allow America's Catholics to celebrate their cultural heritages, the wall sculpture will encourage them to reflect on their spiritual unity, Bransfield said. Anyone who enters the basilica for worship and reflection in the small chapels or in the main church must walk beneath the mammoth work as they leave.

Visitors will see a central dove and seven rays emanating from 46 figures, including Pope John Paul II and Mother Teresa. The figures represent different nationalities but also different social units: families, single men and women, people young and people old. A figure of the Virgin Mary steps toward the dove with one foot and looks down with open arms.

"The rays represent the grace of God, the gift of the Holy Spirit coming down to meet the people," Bransfield said. And the people in turn reach toward God. It's not just nuns, priests and saints who are called to a religious life, he said. "All are called to holiness."

Alexandria filmmaker Martin Doblmeier, who is chronicling the progress of the sculpture for a documentary on the shrine, said he has seen hundreds of churches around the world and could think of no comparable effort to create a "center for art" at the back of a sanctuary.

"Usually the back wall is reserved for the choir loft and for people to come and go," he said. "To create a signature artwork {there} is unique." The sculpture's title comes from Lumen Gentium, a document written at the Second Vatican Council in the 1960s that expresses the council's central theme of unity: "Therefore all in the church, whether they belong to the church or are cared for by it, are called to holiness."

Once the sculpture is in place, Bransfield said, all that remains to complete the Great Upper Church is the laying of mosaics inside three of the five domes that rise more than 120 feet above the floor of the nave.

The goal is to finish all interior work by 2000, when the pope has asked Catholics to make pilgrimages to holy sites around the world, Mukri said. The basilica has applied for designation as "a millennium pilgrimage site" for those who can't afford to go to Rome or to the Holy Land. CAPTION: Monsignor Michael J. Bransfield, rector for 11 years, stands in the Filipino Oratory, which was dedicated in June. CAPTION: Artist's model Stefania Brandt with George Carr, whose design will be translated into marble in Italy. The sculpture is to be installed in the Basilica of the National Shrine of the Immaculate Conception, right, in the fall of 1998. CAPTION: "The Universal Call to Holiness," left, was designed by Silver Spring artist George Carr. Below is an artist's conception of the frieze in place.