Group A streptococcus (GAS) are traditionally classified based on M and T protein antigens. However, many strains do not react with available M antisera and are classified as M nontypeable (M NT). M and T typing, field inversion gel electrophoresis (FIGE), and 5' emm gene sequencing were performed on 24 M NT GAS isolates. FIGE patterns of the M NT isolates were compared by visual inspection and by computer analysis with the patterns of 139 isolates representing 72 M-types of GAS. Seven different FIGE patterns (I-VII) were seen among the M NT isolates. FIGE patterns I and III were identical or closely related to patterns seen with M12 and M22 isolates. The computer analysis determined the following relationships: pattern IV to M5, pattern VI to M61, and pattern VII to M59. Thus, the emm gene sequence correlated with the computer analysis of the FIGE pattern for each isolate. FIGE can be useful to help distinguish clinical isolates of GAS in an epidemiological study.