In order to determining the toxicity of the red alga Gracilaria mammillaris, mononuclear cells from peripheral blood (CMSP), were separated using ficollhypaque technique, which were adjusted to 2 x 10 5 cell/ well and incubated with a metanolic extract from the red alga Gracilaria mammillaris, with and without ferrous sulphate, at 5,0 and 2,5 μg/well. Viability for the whole range with ferrous sulphate was 94 and 95 % and without ferrous sulphate 99 %. The results show toxicity low of the metanolic extract, with and without ferrous sulphate, warranting its use in human, without toxic risk. KEY WORD INDEX