Iron acts as a tumor promoter in T51B cells. Cells treated with (+) or without (-) 0.5 μg/ml MNNG as initiating agent were promoted for an additional 16 weeks in four separate experiments. Media additions during the promotion phase and the number of replicates were: 200 μM ammonium citrate (AmCit), n = 4 (- MNNG), n = 4 (+ MNNG); 200 μM ferric ammonium citrate (FAC), n = 3 (- MNNG), n = 4 (+ MNNG); or 200 μM ferric ammonium citrate,160 μM desferoxamine (FAC, dfo), n = 2 (+ MNNG). Cell transformation was assayed by growth in soft agar as described under Methods. Mean values and standard errors from up to four separate experiments are shown (*p<0.01, **p<0.001 compared to AmCit only controls).