Precultured embryonic chick heart fragments were confronted in vitro with various invasive tumor cell lines (MCF-7/6 human breast carcinoma variant, BW-O-LiI mouse T-cell lymphoma cells and MO4 virally transformed fetal mouse carcass cells) and with a non-invasive cell line (MCF-7/AZ human breast carcinoma variant). Confronting cultures were incubated in the presence of various methanogenic cofactors: Methanobacterium thermoautotrophicum extract, coenzyme F420, FO (7,8-didemethyl-8-hydroxy-5-deazariboflavin), F+ (5'-phosphate derivative of FO), or methanopterin. Histological analysis revealed that the Methanobacterium thermoautotrophicum extract inhibits invasion of MCF-7/6 human cells. At the moment it is not known which factor in the extract is responsible for this inhibition.