700 H. EAGLE AND K. A. PIEZ 
single human cells will grow in the minimal medium of Table I if that minimal me- 
dium is supplemented with whole serum, they do not survive if one uses dialyzed 
serum instead, while large cell populations grow in the dialyzed serum medium as 
rapidly as they do with whole serum. The growth factor supplied by the whole 
serum, and necessary for the growth of small inocula, proved to be explicitly serine™. 
With its provision, a single cell could be grown in 10! vols. of minimal medium. 
The effective serine concentration was on the order of 0.01 mM, and the population 
density at which the serine became unnecessary for growth was on the order of 
100-500 cells/ml (ref. 5). A serine-requiring strain of rabbit fibroblast’? (RT6) 
differed from the generality of mammalian cells only in the population density at 
which the serine requirement disappeared. The inoculum permitting survival growth 
and in an initially serine-free medium was approx. I0 000-50 000/ml, instead of 
100/ml. 

Fig. 4 (a). An electron-microscope photograph of a normal HeLa cell in thin section (from COHEN, 
NYLEN AND Scott’), 
References p. 705 
