Blue Cross and Blue Shield of Virginia announced yesterday that it will help finance a national study of an experimental breast cancer treatment that was the subject of a lawsuit the company lost this year.

In a reversal of its long-standing policy against paying for procedures whose effectiveness have not been proved statistically, the Virginia health insurer has decided to join 21 other Blue Cross plans nationwide in underwriting part of the cost of a research project on the use of autologous bone marrow transplants in fighting breast cancer.

Blue Cross and Blue Shield of Virginia, the state's largest health insurer, is the first Washington area Blue Cross that has agreed to participate in the National Cancer Institute study, which will involve about 1,200 women and be conducted partly at the Medical College of Virginia in Richmond.

In April, a 35-year-old Maryland woman suffering from breast cancer successfully sued the Virginia Blue Cross after the company refused to pay for her autologous bone marrow transplant on the grounds it was experimental.

Jim Goss, a Virginia Blue Cross spokesman, said that in addition to participating in the study, the company has decided to create a $1 million medical research foundation for studies of other experimental medical treatments.