New analogs of diflubenzuron, a benzoylphenyl urea, are tested on their in vitro cytostatic activity against B16 melanoma cells. The following structure-activity relationship was established: substitution by a hydroxylated function at the ortho, meta or para position or by a dimethylamino function at the ortho position of the benzoyl moiety appeared to be necessary for cytostatic activity in vitro. Acetoxy functions at the ortho position or hydroxy functions at the para position of the aniline ring resulted also in active compounds. A number of these benzoylphenyl ureas are selected for in vivo evaluation of antitumor activity on B16 melanoma growing s.c.. Although many of the tested benzoylphenyl ureas delayed tumor growth during the first ten days of drug treatment, only a few increased animal life span. The best results (% T/C) were obtained with compounds 5 (127%), 7 (147%), 13 (135%) and 16 (135%), which all have hydroxylated functions in the benzoyl moiety.