Individual agents within each of the major classes of antibiotics have been shown to have significant adverse effects on spermatogenesis or spermatozoal function in mammals. For humans, infertility or significant alterations in semen parameters have been well documented for the nitrofurans and for patients on sulfasalazine. Other commonly used antibiotics, such as minocycline, have been shown to be toxic to sperm at any concentration. Until further information is available, clinicians must keep in mind that treatment with antibiotics may adversely affect the fertility potential of men. It is possible that some classes of antibiotic agents, such as the penicillins or the quinolones, may have minimal effects on male fertility and maintain the clinical efficacy for patients requiring long-term antibiotic suppressive therapy. Further investigation is needed into the relative toxicity of antibiotics and the mechanisms by which antibiotics affect spermatogenesis and spermatozoal function. A background of the current state of knowledge regarding the adverse effects of antibiotics on male fertility is presented in this review.