Fresh pieces of adult tissue (mainly chicken aorta) (over 40,000 pieces) were treated with various agents and then explanted in a dilute plasma medium. The lag period preceding the first visible growth of fibro-blasts from aortas of 1-yr.-old chickens was normally 3-5 days; of aorta tissue from 5- or 6-yr.-old chickens was about the same, but the average rate of initial growth was 46% faster for the older tissue. Plasmas from the older chickens induced growth 9% sooner than young plasmas. The initial growth rate was 50% faster in 2-yr. plasmas than in 1-yr. plasmas; it was 21% slower in the 5-yr. plasmas than in the 1-yr. plasmas. Trypsin and papain stimulated the growth of adult tissue. The action of trypsin resulted from the proteolytic digestion of the tissue. Embryo extract and spleen extract had little effect on the initial growth of adult tissue. Pituitary growth hormone was slightly stimulating, particularly in the presence of serum. Blood plasma contains a growth stimulant, the A factor, also present in tissues, in lymph, in urine, in serum and in the ultrafiltrate from serum, which reduces the lag period and stimulates initial growth. The dormancy of adult tissue cells seems to involve a balance between the non-diffusible tissue inhibitor, on the one hand, and A factor and proteolytic enzymes on the other hand. The physiological condition of adult tissue cells seems similarly to involve a balance between certain hormone-like controlling agents. There are at least 4 of these in plasma, the A, B, C and D factors. The A factor is a stimulant and is needed by dormant cells. The B factor causes the cells to produce fat granules[long dash]but it does not produce degeneration. The C factor produces degeneration but does not produce fat granules. A D factor produces cohesiveness between fibroblasts.