THE A9 cell is an 8-azaguanine-resistant derivative of the L cell line1. It lacks the enzyme inosinic acid pyrophosphorylase and is thus unable to grow in media such as HAT2 in which endogenous synthesis of nucleic acid is blocked by aminopterin. The A9 line has little ability to grow progressively in vivo. Inocula of 5 × 104 to 2 × 106 cells produced progressive tumours in only 12% of X-irradiated newborn syngeneic C3H mice3. One of these tumours was explanted as a cell suspension into Eagle's minimal essential medium containing 15% foetal calf serum and then subcultivated in this medium with 5% foetal calf serum. At each passage, cells were inoculated into X-irradiated newborn syngeneic C3H or semi-allogeneic C3H×X F1 mice (X designates a number of different allogeneic parents). Between 80 and 90% of the inoculated animals developed progressive tumours. The cell line was therefore designated A9HT (high take incidence). The karyotype of the A9HT line was found to be similar to that of the A9 line, but with a slightly reduced total chromosome number. The modal chromosome number of A9HT was about 53, compared with about 57 for A9 (see ref. 4). A9 and A9HT both had between 20 and 30 bi-armed chromosomes and a number of marker chromosomes in common. A detailed comparison of the karyotypes of the two lines examined by the quinacrine fluorescence technique has been made5. The A9HT line, like its A9 parent, lacks inosinic acid pyrophos-phorylase and is unable to grow in HAT medium.