Semen samples were collected by masturbation under asceptic conditions from men who had sired children within the past 6 mo (group A) and asymptomatic men attending an infertility clinic who had not sired children and whose wives were asymptomatic for infertility (group B). These 109 semen samples were analyzed and cultured for isolation of aerobic and anerobic organisms. Overall, 68% of the specimens had positive bacterial cultures: 54% of the samples from group A were positive and 73% from group B were positive. Mixed bacterial flora were isolated from both groups but in group B they were more varied and present in a higher colony count than in group A. Staphylococcus epidermidis, Staph. aureus, Corynebacterium species, Mycoplasma hominis, and Ureaplasma urealyticum were isolated from group A. Group B revealed these organisma plus Streptococcus pneumoniae type III, Strep. pyogenes group A, Strep. feacalis, Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Pseudomonas aeruginosa, Candida albicans, Bacteroides species, Peptostreptococcus species, and Eubacterium species. Semen samples from group A were of better quality than those from group B. In addition, the antibacterial effect of seminal plasma from group A was greater than that from group B.