We have devised methods facilitating the establishment of continuous cultures of T-cell blasts from patients with acute lymphoblastic leukemia of T-cell type at diagnosis. The cultured cells closely resemble those of the patients at the time of diagnosis with respect to surface markers, karyotype, and T-cell receptor gene rearrangements. Cultured T-cell acute lymphoblastic leukemia (diagnosis) cells (a) are lymphocytes with a convoluted nucleus; (b) have doubling times of 24-48 h; (c) are dependent for growth on interleukin 2; (d) are reverse transcriptase negative; (e) do not form colonies in methyl cellulose; and (f) are clonal with respect to T-cell receptor beta chain rearrangements. Three T-cell acute lymphoblastic leukemia cultures had a normal diploid karyotype, and one had a 6q- deletion which was also present at the time of diagnosis.