Although several reports have demonstrated the acutely toxic and genotoxic effects of landfill leachates in microbial organisms, plants and aquatic animals, the effects of pollutants present in these leachates have not been clarified yet in terrestrial animals. This study mainly aimed to evaluate a potential genetic effect of raw and simulated leachates from Orita-Aperin and Oworonsoki landfills in south-west Nigeria by use of the murine sperm-head abnormality test. These landfills neither have a synthetic membrane liner at the bottom, nor a natural layer of compacted soil with the desired hydraulic conductivity, nor a run-off control system. As a result, the leachates produced are discharged into the environment. Samples designated as Orita-Aperin Raw Leachate (OARL), Orita-Aperin Simulated Leachate (OASL), Oworonsoki Raw Leachate (OWRL) and Oworonsoki Simulated Leachate (OWSL) were analyzed in the sperm-head abnormality test at concentrations (v/v) of 1%, 2.5%, 5%, 10% and 25%. Mice were given 0.5 ml sample per day for five consecutive days by intraperitoneal injection. Each dose group comprised seven mice, and a 5-week exposure period was utilized. The data show that the test mixtures induced a dose-dependent, statistically significant increase (P<0.05) in the number of sperm with abnormal morphology. Physico-chemical analysis of the test samples shows that they contained constituents that are capable of inducing mutation in biologic system. The interaction of some of these constituents with the genetic material in the differentiating cells during spermatogenesis may be responsible for these observations. This is relevant in environmental waste management, and for the assessment of the hazardous effects of the chemicals in landfill leachates.