From examination of some 40 patient strains of Mycobacterium tuberculosis, a statistically very significant correlation (Spearman's rho) can be drawn between the root index of virulence for the guinea pig (D. A. Mitchison) and the ability of the individual strains to elaborate strongly acidic lipids (SAL) in culture. These include both sulfatides (SL) and phospholipids (PL). Since essentially all, if indeed not all, of the virulent and only few attenuated strains are prolific in elaborating SAL, this criterion may be a necessary requirement for the expression of virulence in M. tuberculosis. Tested by chi-square, this premise is seen to be statistically and pragmatically highly significant. We speculate that SL may contribute to the pathogenesis of tuberculosis because of a demonstrable activity directed against host liver mitochondrial membranes (manuscript in preparation) and its synergistic potentiation of the specific toxicity of trehalose dimycolate (cord factor). The activity may also be expressed against phagosomal and lysosomal membranes within macrophages. Because of their strongly anionic character, SL and PL may interact with cationic sites on lysosomal hydrolases with resultant immobilization and/or inactivation of the enzymes. By a similar mechanism, these ionic lipids may alter the activity of bactericidal basic proteins, previously recognized in the lysosomal armamentarium. Since a minor but significant fraction of demonstrably attenuated strains is nevertheless prolific in SL or SAL elaboration, this facility alone is evidently not a sufficient criterion for expression of virulence.