A new cytostatic compound, rhizopodin, was isolated from the culture broth of the myxobacterium, Myxococcus stipitatus. The compound inhibited growth of various animal cell cultures without killing the cells. The ID₅₀, measured by an MTT assay, was 12 - 30 ng/ml, depending on the cell line. Especially cells growing fibroblast-like showed typical morphological changes. They became larger and within hours formed long branching and reticular runners. These morphological changes were irreversible. Rhizopodin suppresses bleb formation in K-562 cells, and therefore could act by interacting with protein phosporylation.