The majority of human peripheral gamma delta T cells express antigen receptors using the V gamma 9 and V delta 2 gene products. Cells of this subset have been previously shown to uniformly recognize mycobacteria regardless of their V-(D)-J junctional sequences in an MHC-unrestricted manner. This reactivity superficially resembles activation of alpha beta cells by bacterial superantigens, which are thought to be presented by monomorphic regions of MHC class II molecules. It is not known whether presentation of the mycobacterial antigen to V gamma 9/V delta 2 T cells is also mediated by class II MHC molecules. In order to examine the similarity between presentation of bacterial superantigens to alpha beta T cells and the presentation of mycobacteria to gamma delta T cells we have studied the role of class II MHC molecules in presentation of the mycobacterial antigen AP-MT to V gamma 9/V delta 2 clones. Activation of gamma delta T cells by AP-MT required direct contact with antigen presenting cells, indicating that an interaction with cell surface molecules on antigen presenting cells is required. Class II MHC molecules were neither sufficient nor necessary for effective presentation of AP-MT to the gamma delta T cells, as transfectants expressing class II MHC molecules were unable to present, whereas cell lines lacking expression of MHC class II molecules could present this mycobacterial antigen.(ABSTRACT TRUNCATED AT 250 WORDS)