We found that human urine suppressed SOS-responses induced by furylfuramide (AF-2), as detected by the umu test using Salmonella typhimurium TA1535/pSK1002. In the present report, we studied the time stability of the SOS-inhibition activity in urine. The diurnal and daily changes of SOS-inhibition in the urine were also observed. Results obtained were as follows; 1) SOS-inhibition activity of the urine remained stable more than one month after the urine was frozen. 2) Individual variation was observed in the SOS-inhibition activity of the urine. 3) Total SOS-inhibition activity of per a day showed relatively small variation during experimental days. 4) The SOS-inhibition activity of urine was higher early in the morning than in the daytime. The activity fell gradually with time in the daytime and showed the lowest value in the evening. Then, it rose again at night. Therefore, it is necessary to collect urine at specific times to avoid the differences caused by diurnal changes in SOS-inhibition activity.