Studies on thymus products. IV. Absence of serum 'thymic activity' in adult NZB and (NZB x NZW) F1 mice.

New Zealand Black (NZB) mice and (B/W) F1 hybrids have a normal level of serum `thymic activity' (TA) at birth but this level decreases prematurely between the third and sixth weeks of life. At 2 months, NZB and NZ (B/W) F1 mice have no significant TA, whereas TA is still at birth's level in six control mouse strains and remains at this level until the fourth to the sixth month. Six weeks after the decline of serum TA, NZB mice show disappearance of theta-positive lymph node rosette-forming cells (RFC) and 2 weeks later, progressive decrease in spleen RFC sensitivity to anti-theta serum (AΘS) and azathioprine (AZ) as in neonatally thymectomized CBA mice.