AbstractSeven UV‐sensitive clones were isolated from the mouse cell lines, FM3A and L5178Y, using Lederberg style replica plating. The UV‐sensitive clones were found at a frequency of 1 in 886 clones of FM3A and of 6 in 420 clones of L5178Y, after treatment with N‐methyl‐N′‐nitro‐N‐nitrosoguanidine, but no UV‐sensitive clones were obtained in the untreated controls. The Do values of freshly isolated clones were 10 to 21 ergs/mm2(mainly 10 to 13 ergs/ mm2), while the original FM3A and L5178Y lines had Do values of 32 ergs/mm2. The UV‐sensitive clones were found to be rather stable for a period ranging from two to eight months after isolation. However, later some of them tended to lose their UV‐sensitivity, in terms of increase in Do values or n values. More stable UV‐sensitive subclones could be isolated from these populations by cloning using the replica plating method. Caffeine was found to potentiate the lethal action of UV‐irradiation on the UV‐sensitive clones as well as