[en] Five far ultraviolet light sensitive (Uvs) strains - CL77, CL89, CL90, CL104 and CL105- were isolated from an auxotrophic strain of Streptomyces clavuligerus (ATCC 27064). When compared to their progenitor, strain CL7, all exhibited a changed mutagenic response to far ultraviolet light (FUV). Strains CL77 and CL104 failed to yield detectable mutants, whereas induced mutants were obtained in strains CL90 and CL89, but only at low FUV doses (up to 20 J m^-2). Strain CL89 was hypermutable at these low doses relative to strain CL7. Strain CL105 gave an increased response in terms of mutants per surviving cell. Caffeine increased the FUV-induced mutation frequency at a particular FUV dose in strain CL7 by up to 3000%. All the Uvs strains showed an altered response to caffeine compared with strain CL7. (author)