Surface configuration of viable cancer cells freshly aspirated from various malignant tumors such as mammary carcinoma, renal carcinoma, malignant melanoma and oat cell carcinoma were described. Various aspects of surface configuration of tumor cells ranging from numerous microvilli, blebs or ridges, to almost entirely smooth surfaces were found. Microvilli were most commonly seen on surfaces of tumor cells from carcinoma of the breast and from malignant melanoma. There were no specific surface features for any of the tumor cells studied except for oat cell carcinoma. This is in contradistinction to the configuration of cancer cells in effusions wherein most cancer cells are covered by dense, irregular microvilli.