Studies comparing the post-treatment behavior of fibroblasts from patients with ataxia telangiectasia with that of fibroblasts from control donors showed then similar behavior after either methyl methane sulfonate (MMS) or ultraviolet (UV) treatment, but different behavior (decreased colony forming ability and usually decreased DNA-repair synthesis) after N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) treatment. Similar studies using a number of human tumor cell strains showed many of these to be extremely sensitive to MNNG-produced killing, somewhat sensitive to MMS and BCNU (bis-chloroethyl-nitrosourea) killing, but not to UV-produced killing.