Sonicates derived from Mycobacterium tuberculosis suppressed lymphocytes proliferation. Pulsing of monocytes with mycobacterial sonicates resulted in the release of high molecular weight lipids. Both these lipids and those prepared by column fractionation of mycobacterial sonicates suppressed lymphocyte blastogenesis.This effect was due to the activation and not the proliferation of CD8+ lymphocytes by the lipid containing mycobacterial fractions of Mr>200kDa that could be obtained in vitro by column fractions.