Abstract: The effect of several ratinoids on colony-forming ability ofmouse B16 melanoma cells were compared, using both liquid and soft-agar culturetechniques. RⅠ, RⅡ and R81001 all synthesized in this Institute were shown to haveno effect on the number of colonies formed by adherent cells in liquid medium atconcentrations of 3.3×10^-6 mol/L to 1×10^-8 mol/L. Only slight inhibition wasobserved when RA 3.3×10^-6 mol/L was used. In the soft-agar medium, however,all four compounds showed significant inhibitory effect on colony formation even atthe lowest concentration (1×10^-8 mol/L). The activity of these compounds in termsof their inhibitory effect was in order of RA>RⅡ>R81001>RⅠ. Since the abilityof colony formation in soft-agar medium is one of the characteristics present inmaligant cells.which distinguishes them from normal cells, the soft-agar colony-formingassay of B16 cells may provide a modal for screening compounds with potentialactivity in cancer prevention, and also, in comparing the activity between retinoidsand the like.