A Cu(II)-diethyldithiocarbamate modified carbon paste electrode was found to be highly selective for cysteine accumulation. Of the simple amino acids, only histidine interfered at 10 times the concentration of cysteine. Thiourea and thiocyanate ion interfered only when their concentrations were 30 times and 10 times greater than that of cysteine. The detection limit using differential pulse anodic stripping was 2.00 x 10−6mol 1−1cysteine and the determination of 5.00 x 10−5mol 1−1cysteine gave a precision of 3.95% (r.s.d., n = 8).