Fibrin overlay methods are described which can detect the plasminogen activator produced by single transformed cells or small colonies of transformed cells. These methods were applied to malignant cells derived from humans, mice, hamsters, rats, and chicks. The lysis observed was plasminogen dependent. Transformation of chicken cells by Rous sarcoma virus was detected 4 days after infection. The number of lysis zones produced was proportional to the virus inoculum and was identical to the number of morphologically determined foci. These methods may also have application in model systems for scoring transformation by chemicals. Transformed mouse and chicken cells were detected at the single cell level and the number of lysis zones produced was dependent on the number of cells present, the time of incubation, and the concentration of plasminogen.