Crossed immunoelectrophoresis was employed in the study of precipitating antibodies against Staphylococcus aureus in sera from 46 patients with S. aureus septicaemia. The occurrence and titres of antibodies were compared to previously reported findings for normal persons. Ten infection-specific precipitins were detected. However, most of these precipitins were detected in low titres and could only be detected during convalescence. When antibodies against a single antigen were measured, titres above the normal level were found in 39.1% - 65.2% of the patients. When the total number of precipitins in each serum was compared to findings in serum from normal persons, 54.3% of the patients had a higher value. Most promising was the precipitin score, considering both the number of precipitins and their titres. This score showed an early rise and was increased above the normal level in 82.6% of the patients. The antibody response against individual S. aureus antigens varied from person to person both concerning titres and time course.