The serum zinc, calcium and magnesium concentration as well as the possible relationship between these elements was determined by means of atomic absorption spectrophotometry in 150 patients with prostatic, lung, liver and breast neoplasms as well as with chronic leukemia, and in 80 normal subjects. These determinations showed that zinc concentration is increased in almost all the forms of cancer except prostatic and lung cancer in which the zinc values were below the lowest normal limits. From this study it also resulted that high zinc values favour the development of cancer. The antagonic relationship between zinc and calcium or magnesium in neoplasias proves the protective role played by the latter elements in the forms of cancer in such cases.