1. Five types of human tissue when freshly explanted in vitro were found to have varying but limited growth potential in contrast to the autonomous and apparently unlimited growth of the transformed cell.

2. A requirement for an external source of inositol or glutamine could not be demonstrated for the slowly multiplying primary amnions in contrast to the rapidly growing transformed cells which degenerated in the absence of either metabolite.

3. Unlike the transformed cells, the primary amnion cells fixed an insignificant amount of CO₂; when infected by the Coxsackie virus, however, a requirement for CO₂ in the formation of more virus became readily demonstrable.

4. The biggest difference in the incorporation of various C¹⁴ substrates occurred in the nucleic acid fractions.

5. The difference in the incorporation of glycine into the protein fraction was very small.

6. The utilization of glucose, ribose, and xylose as well as the oxidation of lactic acid into CO₂ by the primary amnion cells occurred at a rate much lower than that of the transformed cells.

7. The cytotoxic dose of 5-fluorodeoxyuridine for the primary amnion was at least 100 times that of the transformed cells although growth inhibition was readily demonstrated at 0.1 µg. per ml. for both types of cells.

Based on these observations, several hypotheses are put forth for consideration.