Malignant melanoma of the conjunctival region was induced in the golden Syrian hamster. Inoculation of this lesion with Calmette-Guerin bacillus (BCG), 48 hours after injection of 10(3) tumor cells, either slowed, limited, or completely prevented the growth of the tumor in 56% of the treated eyes. In 98% of the control eyes, malignant melanoma grew rapidly and extensively.