Landfills are used to dispose municipal solid wastes, and although on-site recycling in these places is an extensive practice in Latin America, diverse pollutants are incorporated into the leachates. The objective of this work was to establish relationships between composition and toxicity of leachates from the landfill of the city of Cartagena, Colombia. Leachates were characterized measuring Cd, Ni, Hg, Mn, Cu, and Pb concentrations, and physicochemical parameters including pH, conductivity, chemical oxygen demand (COD), and hardness. Bioassays were conducted diluting with synthetic sea water, recording toxicity against Artemia franciscana as median lethal concentrations (LC50 values) after 24 and 48 h exposure. Average LC(50) values oscillated between 3.20% and 39.33% (v/v). Multivariate analysis showed that toxicity was dependent on Cd and COD. The slope of the concentration-response curve correlated with Ni concentration independently from toxicity. Results suggest toxicity of these leachates depends on Cd concentrations associated with organic matter, this effect being modulated by Ni.