The University of Michigan endometrial carcinoma cell line UM-EC-1 was derived from a poorly differentiated endometrial adenocarcinoma of a 66-yr-old white female. Cell cultures were started using both tumor explants and a cell suspension obtained from collagenase-treated tumor tissue. The collagenase-derived cell suspension gave rise to monolayer cultures which grew rapidly from the outset. This subline of UM-EC-1 has now been subcultured more than 50 times. Cells derived from the tumor explants grew more slowly initially, but after a lag phase of 5 to 6 wk, this subline also exhibited rapid logarithmic growth and reached the same growth rate as that of the collagenase-treated cells. The explant subline has been subcultured more than 37 times. The doubling time of both sublines is 24 h under optimal growth conditions. The karyotype of both cell cultures is 43, XX, inv(1)(p32q42), -4, +der(8) t(8;12)(p23.1;q22), del(9)(q11), -13, -13, +t(13;13) (p13;p13), del(18)(q), -19, -22, -22, +t(22;22)(p11;p11). The net result of the chromosome losses and rearrangements was monosomy 4, duplication 8p23.1----qter, deletion 9q11----9qter, duplication 12q22----qter, deletion 18q, and monosomy 19. The t(13;13) and the t(22;22) were dicentric by C-banding. Virtually all of the chromosome changes were stable in multiple passages except that there was mosaicism for chromosome 13. Some cells contained a single copy of 13 and others had t(13;13). The available evidence indicates the t(13;13) is an isochromosome. UM-EC-1 cells produced tumors histologically similar to the original tumor in male, female, and ovariectomized female athymic mice. UM-EC-1 cells express human class I histocompatibility antigens as assessed by binding of antibodies to nonpolymorphic HLA and beta-2-microglobulin antigens. Blood group antigens A and H were absent although the patient is blood type A and these antigens are normally expressed in endometrial glands. A rearrangement involving the region of chromosome nine that carries the ABH locus may be related to the absence of blood group antigen expression by these cells. The E7 membrane antigen, the locus for which resides on the short arm of chromosome 11, was expressed strongly which is consistent with the presence of two intact copies of chromosome 11 in these cells.