The comparative detergency of sodium dodecyl sulfate (SOS) solution near the first critical micelle concentration (CMC) was measured by means of a ¹³¹I-labelled castor oil as a soil. More than 95% radiochemical purity of ¹³¹I-labelled castor oil was obtained using potassium Iidide as a carrier. Polyester test fabric was soiled with ¹³¹I-labelled castor oil, and washed in a conventional washing apparatus mounted on appropriate devices. Fabric radioactivities were measured before and after washing by a scintillation counter. Near the first CMC, the detergency of SOS was increased with decreasing of surface tension of SOS. It was also shown that ¹³¹I-labelled castor oil was useful for studying the detergency of SDS.