Evaluation of joint toxic action of metal ion mixtures is one of the priority research areas due to the simultaneous occurrence of metals in the environment and the health risk they posed to humans and the environment as a mixture. Individual and composite mixture acute toxicities of arsenic (As), cadmium (Cd), mercury (Hg), and lead (Pb), which are among the top eight toxic chemicals, were characterized at varying concentrations. MCF 7 cell lines were exposed to individual and composite mixtures containing the four metal ions in the proportion of their EPA's MCL for 24 hours, and the concentration-response data were generated spectrofluorometrically. Acute toxicities were estimated based on the uptake of fluorescence diacetate dye. Toxicological interactions among the four metals were profiled, based on computed interactive index. Results demonstrated that the toxicity of each of the metal ions was enhanced in the composite mixture, and the metals demonstrated differential interactions in a concentration dependent manner. Lead, the least toxic among the four metals, showed the highest enhancement (23-to 64-fold) in toxicity when in the mixture. Interaction among the four metals was largely additive although there was slight departures form additivity at the two extremes of the concentration range.