Three groups of 24 C57BL/6J black mice were studied. One group was implanted with B16 malignant melanoma, another was implanted with mammary adenocarcinoma, and the third was not given tumor implants. After 14 to 17 days, the mice were given injections i.v. of technetium-99m sulfur colloid and killed 30 min later. Organs were weighed, and radioactivity was counted. The ratios of specific radioactivities of the spleens to those of the liver were higher only in the group of mice bearing malignant melanomas. This finding suggests that the "hot spleen" phenomenon observed in humans with malignant melanomas may be due to increased specific activity rather than increased splenic volume.