The oral hypoglycaemic drug 1-(p-chloroben-zenesulfonyl)-3-propylurea (chlorpropamide) was examined for clastogenic properties using human lymphocytes with a microsomal (S9 mix) activation system. Lymphocytes, exposed to the S9 mix only as well as to the solvent DMSO, yielded chromosome aberration levels within the background range. The sensitivity of the S9 mix could be demonstrated using 2-[bis(2-chloroethyl)amino]tetrahydro(2H)-1,3,2-oxazaphosphorine 2-oxide (cyclophosphamide) as a positive control, which only displays clastogenic properties after metabolic activation. Chlorpropamide, however, did not induce structural chromosome aberrations under the in vivo conditions tested in this study.