Tumor cells from 10 patients with Epstein-Barr virus-positive endemic Burkitt's lymphoma (BL) have been examined for cell surface phenotype, both at the biopsy stage and during BL cell line outgrowth in vitro, the cultures being followed for up to 150 passages. In all 10 cases, the biopsy cells showed coexpression of the common acute lymphoblastic leukemia antigen (CALLA) and of the BL-associated glycolipid antigen (BLA) with no accompanying expression of several "lymphoblastoid" cell surface markers defined by selected monoclonal antibodies. During cell line establishment and in vitro passage, the individual BL cell lines showed different degrees of progression toward a more "lymphoblastoid" cell surface phenotype, some even losing CALLA and BLA expression while retaining the chromosomal translocations indicative of their malignant origin. This differential capacity for phenotypic progression in vitro explains much, if not all, of the heterogeneity of the BL cell phenotype apparent from many previous studies with panels of long-established lines. Such heterogeneity in vitro belies the true homogeneity of the tumor cell phenotype in vivo.