Mutants of the thienamycin producer, Streptomyces cattleya NRRL 8057 were induced by ultraviolet irradiation at 254nm. Some characteristics of the mutants (which included growth rates at different temperatures, sensitivity to different antibiotics, nutrient utilization, auxotrophy, reaction to cobalt, sodium chloride and pH) were examined. Using numerical taxonomy techniques, it was possible to define sub-populations of high titre antibiotic producers. Cluster analysis and principal components analysis were used to identify clusters of high titre antibiotic producers. Both analyses gave similar results, identifying the same clusters of high titre producers. Discriminant analysis was used to characterize the high titre producers. The characters which differentiated the high titre producers were growth rate at 20 C, starch hydrolysis, resistance to neomycin, erythromycin and cobalt. These selected characters were used to design selective media for the isolation of high titre antibiotic producing UV radiation induced mutants of S. cattleya. Individually and in paired combinations, the selective pressures applied in the media selected for high titre producing mutants. In particular, media containing paired combinations of neomycin, erythromycin and cobalt selected for mutants with high antibiotic productivity. Cobalt was identified as the single factor which selected for the highest number of mutants producing detectable levels of antibiotics and high amounts of biomass. This application of numerical taxonomy to the selection of high titre antibiotic producing strains is a novel approach to screening for such strains. Including this technique in a strain development programme would offer the advantages of a high through put, increased probability of selecting for the desired mutants, making it unnecessary to carry out repeated bioassays and thus, saving time and effort.