Abstract: This invention relates to a composition for use in a method for determining the presence or amount of an analyte in a sample comprising a labeled constituent consisting of an aqueous carbon sol which is stable in the absence of a stabilizing substance. The colloidal carbon particles have directly conjugated to their surface a binding component which specifically binds to the analyte and the resulting analyte/carbon particle complex is used as an indication of the presence or amount of analyte in the sample. The grade of carbon used satisfies the condition V>0 wherein V is a linear predictor value according to the formula V=-138.954-0.987.times.DBP+15,609.times.VC+3.994.times.PPD, wherein DBP is the dibutylphthalate adsorption in ml/100 g, as determined according to DIN 53601; VC is the volatile content in %, as determined according to DIN 53552; and PPD is the average primary particle diameter in nanometers.