Bioassays were conducted to observe the responses of species of Micrococcus ann Pseudomonas to the metals zinc, lean, copper and manganese, added singly or in combination to a crude oil/mineral salts medium. Bacterial density decreased with increasing metal ion concentration. A 50% reduction (LC50) in the bacterial density of Micrococcus sp. occurred on the addition of either 0.46 mu g zinc/ml, 1.41 mu g lead/ml, 11.25 mu g copper/ml or 28.2 mu g managanese/ml. For Pseudomonas sp., the corresponding values were 0.43 mu g zinc/ml, 2.80 mu g lead/ml, 6.30 mu g copper/ml or 317.0 mu g manganese/ml. Of the metals tested, only zinc was found in higher concentrations in the oil contaminated site than the LC50 value obtained.