Biocidal preservative agents are important in preventing the growth of pathogenic and nonpathogenic organisms in a variety of personal care, household and industrial products. Nearly all effective, commonly used preservative materials possess skin sensitization potential. In a series of 13 prophetic human repeat insult patch tests involving a total of 1450 subjects, no skin sensitization was induced at concentrations of 10 ppm, 6 ppm or 5 ppm (1121 subjects) or at 15 ppm (200 subjects) of a new isothiazolinone preservative mixture (Kathon CG). Delayed contact hypersensitivity was induced in 1 of 84 subjects at 12.5 ppm and in 2 of 45 subjects at 20 ppm. These results add further support to the previously reported conclusion that use of this new biocide at the very low concentrations required for effective preservation of rinse-off products involves an extremely low risk of clinical dermatoses.