Synthrapol SP was specifically developed by Imperial Chemical Industries for the purpose of soaping off Procion dyes. The "SP" stands for "soaping Procions". It is extremely popular among hand-dyers in North America and Europe.

Generic equivalents available for the hand dyer's market include Dharma Professional Textile Detergent and G&S TNA Soap. Undoubtedly a number of different brands of similar products are available for use in the textile industry.

Almost any textile detergent with a neutral pH will work. Only small quantities are required. More important than the choice of detergent is the temperature of the water. Hotter water is far more efficient than cooler water for the purpose of removing unattached fiber reactive dye.

See "What is Synthrapol?", a question in the hand-dyeing FAQ on this site.