Recipient mice injected with group A β-hemolytic streptococcus (GABHS) sera have less territoriality and more passive behavior in a social interaction test. Naive male SJL/J mice were injected with either pooled GABHS or phosphate-buffered saline (PBS) donor sera, or immunoglobulin G (IgG)-depleted GABHS or PBS donor sera. Social investigation (
a) and defensive-escape ( b) behaviors of mice were tested in a social interaction (resident–intruder) paradigm. Resident mice injected with GABHS donor sera tended to have reduced social investigation and increased defensive-escape behaviors as compared to control mice injected with PBS donor sera (social investigation, Mann–Whitney U; * P=0.066; defensive-escape, Mann–Whitney U; ‡ P=0.108). Social investigation behaviors were higher and defensive-escape behaviors lower in resident recipient mice injected with GABHS donor sera depleted of IgG relative to the levels observed in mice receiving nondepleted GABHS donor sera, but did not achieve significance (social investigation, Mann–Whitney U, § P=0.079; defensive-escape, Mann–Whitney U; # P=0.39). No differences were observed in social investigation or defensive-escape behaviors in the control group injected with IgG-depleted PBS donor sera as compared to resident recipient mice injected with nondepleted PBS sera. Height of box plot shows interquartile range; horizontal line, median; error bars, range; circles, outliers.