All members of the MDC family of integral membrane proteins contain a metalloproteinase-like domain, a disintegrin-like domain and a cysteine-rich domain. They have been identified in a wide range of mammalian tissues and many are abundantly expressed in the male reproductive tract. We have previously cloned and sequenced several MDC transcripts expressed in the testes of the macaque (Macaca fascicularis). Using polyclonal antisera we have localized a number of these MDC proteins (fertilin alpha, fertilin beta, tMDC I, tMDC II and tMDC III) to spermatogenic cells and demonstrated that they are processed as spermatozoa pass through the epididymis, yielding proteins that retain their disintegrin domain on mature, fertilization-competent cauda spermatozoa. In rodents, two of these MDC proteins (fertilin beta and tMDC I) have been implicated in egg recognition, mediated by a disintegrin-integrin interaction. It is possible that the additional MDC proteins expressed on macaque spermatozoa may also be involved in this process.