Proteins form the membranes and culture media of nine, human T lymphoblastoid cell lines and of the myeloid line K562 were analyzed after 35S-methionine metabolic labeling, by electrophoresis in SDS polyacrylamide gradient gels. A monotonous consistency was present among these cell lines in their synthesis of membrane proteins, with only few variations in the expression of most proteins. In contrast, the synthesis of culture supernatant proteins was found to differ substantially among the cell lines. Proteins of 35,000 and 36,000 daltons were most prominently synthesized by cell line 1301. Proteins of 50,000 daltons were abundantly synthesized by SKW 3 and 8402. Proteins of 65,000 daltons were best synthesized by Jurgat, 1301 and CEM. Few of the membrane proteins which were abundantly synthesized by mitogen-activated normal T lymphocytes were expressed on the cultured cell lines. The view was proposed that cultured T lymphoblastoid malignancies do do not represent static vignettes of stages of differentiation of normal T lymphocytes.