Interferons (IFNs) are a group of cytokines exhibiting antiviral, antiproliferative and immunoregulatory properties. The principal stimulus for the synthesis of IFNs is the presence of viral double-stranded RNA, although rare examples of constitutive synthesis have also been described. The aim of the present study was to determine IFN-α-like biological activity in the seminal plasma, follicular and amniotic fluid, embryo culture medium, and fetal blood obtained from patients without apparent viral or bacterial infections. Interferon-α-like biological activity was determined by a standard cytopathic effect inhibition bioassay. The study included two groups of patients. The first group consisted of 30 married couples participating in the programme for assisted reproduction and the second group consisted of 23 patients scheduled for prenatal diagnosis (15 for amniocentesis and eight for cordocentesis). The seminal plasma of infertile men (asthenozoospermia, oligoasthenozoospermia) contained a high titre of IFN-α-like antiviral activity. Asthenozoospermia was diagnosed in men with a normal sperm concentration but less than 50% progressively motile sperm and oligoasthenozoospermia was diagnosed in men with a sperm count less than 1 × 10⁶ mL^-1. Despite slightly higher antiviral titres in the seminal plasma obtained from asthenozoospermic patients, no clear association between IFN-α-like biological activity and sperm concentration was found. Interferon-α-like biological activity was found in all samples of follicular and amniotic fluid and in fetal blood of patients with intrauterine growth retardation and trisomy 18. Antiviral titres from seminal plasma and follicular fluids were significantly higher compared with amniotic fluids and fetal blood. Embryo culture medium did not contain IFN-α-like biological activity. Our results demonstrate that IFN-α-like activity in biological fluids is relevant for reproduction, even in the absence of infection.