The t haplotypes, mutant forms of the proximal third of mouse chromosome 17 (the t complex), contain factors that contribute to defective sperm function in fertilization. Males carrying two t haplotypes (tx/ty mice) are sterile; their sperm have very poor motility and are unable to penetrate zona-free eggs. Although males carrying one t haplotype (t/+) are fertile, genetic evidence suggests that the sperm carrying the normal form of chromosome 17 (+t) are dysfunctional in fertilization, and some or all sperm have abnormal motility. Some of the same genetic factors that cause sterility in tx/ty males probably contribute to the dysfunction of +t sperm from t/+ males; however, it is unclear which steps in gamete interaction are defective in sperm from t/+ males, or whether the defects are similar to those observed in sperm from tx/ty males. We have developed a unique low sperm:egg ratio IVF assay for sperm function in fertilization. Using this assay, we have shown that tw5/+ sperm are less able than congenic +/+ sperm to penetrate the zona (probably due to their abnormal motility) and to penetrate the zona-free oocyte. Since tw5/tw32 sperm are unable to complete these same two steps in sperm-egg interaction, these specific deficits could be involved in both transmission ratio distortion and sterility. We have also shown that tw5/tw32 sperm are deficient in their ability to bind to the zona and to the oolemma. These results suggest that t haplotypes contain loci which affect a number of sperm functions and thus could be a rich source of genes important for sperm-egg interaction.