Methods and devices for electrochemical detection of a specific binding pair member utilizing a microsphere with an incorporated electroactive marker, wherein a member of the specific binding pair to be detected is bound, directly or through one or more intermediates, to the microsphere. Multiple specific binding pair members may be detected by use of electrochemically distinguishable electroactive markers. Microspheres with incorporated electroactive markers may include one or more functional groups for binding members of specific binding pairs, and are preferably insoluble in aqueous solvents but soluble in selected organic solvents.