A chemiluminescence method was used to study opsonization of group A Streptococcus (GAS) of serotype T1M1 in serum samples ("sera") obtained from Swedish patients with invasive and noninvasive GAS infection and from healthy blood donors. Acute-phase serum samples ("acute sera") generally demonstrated low ability to opsonize the patient's own GAS isolate, regardless of clinical manifestation. Only approximately 15% of serum samples obtained from healthy blood donors demonstrated high opsonic activity against a standard T1M1 strain. Opsonization of 62 T1M1 isolates (obtained during 1980-1998) by a single immune serum sample showed considerable variation; this indicates that high opsonic immunity may develop only against the infecting isolate or identical clones. T1M1 GAS isolated from 1987 through 1990 were better opsonized by the immune serum sample than were isolates obtained before 1987 or after 1990, a finding that suggests a temporal change of the surface properties that affect opsonization.