The marine tetramic acid (=1,5-dihydro-4-hydroxy-2H-pyrrol-2-ones) derivatives melophlin P, Q, and R (1p-1r, resp.) were synthesized for the first time in only four steps. Together with the congenerous melophlins A-C and G, they were also tested for antimicrobial and cytotoxic effects. Melophlins B, C, P, Q, and R, which share a 5-Me residue, showed some antibacterial activity, mainly in Gram-positive bacteria. Melophlins B, C, and R, which have Me-branched 3-acyl side chains in common, inhibited the growth of cells of human KB-3-1 cervix carcinoma, A-498 kidney carcinoma, and U-937 leukemia with IC(50) values <10 muM. They were similar in activity to cisplatin. Melophlin Q, also Me-branched, was astoundingly specific in inhibiting A-498 kidney cancer cells, while melophlin P inhibited U-937 leukemia cells particularly well. The position of the Me branch is decisive for the magnitude of the antiproliferative effect of the melophlin couples B/C and R/Q.