Thin-layer chromatography (TLC) was compared with seroagglutination for subtyping of Mycobacterium avium complex (MAC) bacteria. Seventy-five significant MAC isolates from patients with AIDS were typed by both methods and 36 isolates, judged to be clinically non-significant, were examined by TLC only. Overall, 75% of isolates tested were typable by seroagglutination and 91% by TLC; the results correlated between the two except for minor discrepancies. Serovars 1, 8 and 21 and mixed serovars 1-21 and 1-8-21 were common among isolates from AIDS patients and together represented 83% of isolates compared with only 36% in the non-significant group (odds ratio 8.4; 95% confidence interval 3.4-23.3). This difference remained significant after exclusion of serovar 41 (M. scrofulaceum), which was the commonest isolate (28%) in the non-significant group but was not isolated from patients with AIDS. TLC is useful to supplement seroagglutination for subtyping of MAC. Further study is required to determine whether apparent differences between isolates from patients with AIDS and from other sources reflect differences in virulence or in environmental prevalence of MAC subtypes.