The invention relates to novel polycyclic alkaloids of formula ##STR1## in which R.sup.1 represents hydrogen or hydroxy, R.sup.2 represents hydrogen or acyl and A and B, independently of one another, each represents C.dbd.O or C--OH, and wherein the dashed line represents a C.dbd.C double bond at the position of the middle bond when A or B represents C.dbd.O, or at the position of the two outer bonds when A or B represents C--OH, a process for the manufacture of these compounds by fermentation of a novel microorganism of the species Myxococcus xanthus, the novel microorganism itself, pharmacetical preparations that contain the novel compounds, and the use of the novel compounds as antibiotics and as tumour-inhibiting agents and for the manufacture of pharmaceutical preparations.