Various lipopeptides representing the N-terminal part of the cytochrome subunit of the photosynthetic reaction centre from the purple bacterium Rhodopseudomonas virdis were prepared by solid-phase peptide synthesis. These lipopeptides consisted of a S-[2,3-dihydroxypropyl]-cysteinyl (Dhc) residue N-terminally coupled to the nonapeptide FEPPPATTT. Different numbers of palmitoyl (Pam) chains were attached to Dhc via ester and/or amide bonds. The lipopeptide Dhc(Pam)2-FEPPPATTT containing two ester-bonded palmitoyl residues and a free N-terminus was a potent polyclonal activator of murine (BALB/c) spleen cells at subnanomolar concentrations. The lipopeptide Pam-Dhc(Pam)2-FEPPPATTT containing three palmitoyl residues, the two-chain lipopeptide Pam-Dhc(Pam)-FEPPPATTT containing one amide- and one ester-bonded palmitoyl residue, and the N-terminally elongated lipopeptide SLVAG-Dhc(Pam)2-FEPPPATTT were less active. The nonapeptide FEPPPATTT and the decapeptide Dhc-FEPPPATTT were only marginal splenocyte activators, even at concentrations as high as 1 microM. Thus, lipopeptide Dhc(Pam)2-FEPPPATTT constitutes the first potent splenocyte stimulation Dhc-lipopeptide described so far that contains only two fatty acid residues.