MPB is a versatile thiol-specific biotinylating reagent. It is specific for sulfhydryl groups or reduced disulfide bonds and will not label proteins lacking a free sulfhydryl group at a pH between 6.5 and 7.5. This probe can be used in combination with the appropriate avidin- or streptavidin-conjugated markers (i.e., fluorescent, enzyme-conjugated, etc.). MPB can be used to detect protein sulfhydryl groups on dot blots with sensitivities in the femtomole range. MPB can be utilized for cell sorting, enzyme immunoassay, protein blotting, and various cytochemical procedures.