1. A study of 1,000 liver photoscans was done emphasizing the reasons for false positives and false negatives. 2. Common causes for false positive scans were: (a) the gallbladder fossa, (b) pressure defects of tumors, (c) the fossa for the inferior vena cava, (d) the porta hepatis, (e) vertebral pressure defect in the midline, (f) thinning out of the left lobe of the liver, (g) pressure defect from the right kidney, and (h) anatomic variations in size, shape, or position of the liver. 3. I131 rose bengal photoscans following false positives by the colloids (Au198 or Tc99m) prevented many errors in diagnosis. 4. False negative scans, or missed diagnoses, were proven at abdominal operation or autopsy on all metastases below 3 cm. in diameter. Numerous nodules 4 cm. in diameter, several 6 cm. in diameter, and a few larger metastases were missed on the photoscan. 5. The gamma camera, or the rectilinear scanner, attached to a computer for quantitative recording of counts, and a larger dose of isotope for more rapid scans, will be used more often in the future and should improve our present results.