Effect of sFn pre-treatment on monocyte cytotoxicity against tumor cells. Calcein AM labeled PBM incubated with A375 cells after pre-treatment of A375 with or without sFn and pre-treatment of PBM with or without sFn prior to assay. sFn pre-treatment of monocytes was slightly inhibitory (P <0.05 compared to untreated control; n = 3). Significantly greater inhibition was observed when A375 cells were sFn pre-treated (P <0.01 compared to untreated and to monocyte treated cells; n = 3). Maximal inhibition of PBM cytotoxic activity occurred when both cell types were treated with sFn (P < 0.01 compared to untreated, monocyte treated or A375 treated cells: n = 3).