The presence of anti-cardiolipin antibodies (aCL) in women is an established risk factor for fetal loss, but these autoantibodies had not previously been investigated in the male partners of recurrent aborters. The prevalence of aCL in 144 male partners of couples with a history of recurrent miscarriage was 4.2%, which compares with 2.8% for 177 female partners of such couples, 5.9% of 254 blood donors (128 male, 126 female) and 0% of 74 women of good obstetric history sampled in the puerperium. In recurrent miscarriage couples, the presence of aCL in the women was associated with a poorer history and outlook, but these associations were not evident in couples where the male partner had aCL. The aCL of male partners also differed from those of their spouses in being less likely to show higher activity towards phosphatidylserine than towards phosphatidylcholine. A particular sub-population of aCL may be strongly represented in the male partners, the clinical significance of which remains to be established.