A capacitive immunosensor was fabricated by immobilizing monoclonal antibodies on a polytyramine-gold nanoparticle layer. The immunosensor was used to quantify microcystins produced by Microcystis aeruginosa; MC-LR, DmMC-LR and MC-LA, and further identified by LC- MS/MS. The results show that cumulative determination of microcystin variants is possible with this immunosensor.