In an attempt at prenatal diagnosis of fetal infection after primary rubella infection before 18 weeks of pregnancy fetal blood was taken by direct puncture under ultrasound guidance at 20-26 weeks of pregnancy from eighteen patients. Total IgM was assayed by radial immunodiffusion and rubella-specific IgM by IgM capture immunoassay. Rubella-specific IgM was detected in twelve of the eighteen fetuses. The parents decided whether to continue or terminate the pregnancy on the basis of the time of onset of maternal rubella. All six pregnancies in which rubella occurred before 12 weeks of gestation were terminated. Of the six with rubella after 12 weeks two were terminated. Among the six fetuses negative for rubella-specific IgM, five had no rubella-specific IgM at birth and no persistent IgG but one was found to be infected at birth; this false-negative diagnosis was due to sampling too early during pregnancy.