The plaque-reduction, serum-dilution neutralization method satisfied most of the important criteria for efficient measurement of antibodies against Tacaribe-group viruses. This technique is type specific, economical to perform both in time and expenditure of reagents (0.025 ml of serum provides quantitative data), does not require special additives such as complement, is highly reproducible and easy to read. In practice, a team of two technicians easily tested 600 sera/week. By reversing the basic procedure and diluting virus rather than serum, it is simple to identify Tacaribe-group isolates.