The effect of slag composition on lead losses to slags and on the behavior of associated accessory elements in lead blast furnace smelting has been studied by the Bureau of Mines. Equilibrations of 30 g of lead and synthetic slag doped with cu, in, tl, and sb at 1,200 deg c in co-co2 atmospheres revealed the importance of the feo:sio2 ratio and the basicity ratio, (cao+mgo):sio2, of the slag. Lead losses to slags increased with an increasing feo:sio2 ratio and decreased with an increasing basicity ratio. Copper and indium distribution coefficients, lx - wt-pct x in bullion/wt-pct x in slag, increased from 15 at the lower basicity ratios to near 60 at higher basicity ratios. Ltl values increased from 20 to 150 as slag basicity increased. Lcu, lin, and ltl were relatively independent of the feo:sio2 ratio, but increased with greater amounts of accessory elements in the system. Antimony distribution alone was shown to be strongly affected by feo:sio2 ratio, although data were scattered. Lsb values ranged from 20 to 800 and decreased sharply with increasing feo:sio2 ratios. The basicity ratio and the level of accessory element load did not have any noticeable effect on lsb.