The cervical carcinoma cell line, MEI80 cells were treated with tanshinone (Tan) or retinoic acid (RA) in DMSO (final concentration 0.02%, V/V) on 4 successive days. The cells treated with the same concentration of DMSO alone served as control. Morphological studies with light and transmission electron microscopy showed that the cells treated with both Tan and RA became welldifferentiated. The cellular growth and proliferation were suppressed (as revealed by cell counting. [³H]-thymidine uptake and colony-forming assay). The number of nuclear organizer regions (AgNORs) in cells reduced and the distribution type returned nearly to normal type. The tumorigenicity in nude mice was reduced. The cell RNA dot hybridization showed that the expression of c-mye and c-Ha-ras mRNA was inhibited markedly. All above restllts showed that Tan and RA could reverse some malignant phenotype and possessed differentiation inducing activity on MEI80 cell line. No significant difference was observed between the cells treated with Tan and RA.