Abstract: A reagent useful in immunoassays, comprising a direct particulate level co-sensitised with a specific binding agent having specificity for an analyte or analyte analogue and with a non-specific protein which can participate in a control reaction with another specific binding agent which does not bind to the first specific binding agent nor participate in the formation of a complex by means of which detection of the analyte or analyte analogue is accomplished. Preferably the first specific binding agent is an antibody raised in a first species and the non-specific protein is an immunoglobulin from another species. Optionally, the reagent additionally comprises a second population of the direct particulate label sensitised solely with the non-specific protein.