Further mapping of the mouse T-cell factor specific for poly(Try,Glu)-polyD-LAla--polyLys is reported. It is shown to be a product of the I-A subregion of the H-2 complex by the use of antisera either raised specifically against or made specific, by absorption, for different regions of the H-2 complex. The factor cooperates across allogeneic barriers, e.g., when factor produced by one strian is combined with bone marrow cells of other H-2 incompatible strains.

L ], 1 a n d s u b s e q u e n t l y c u l t u r e d briefly in vitro, release a soluble factor c a p a b l e of r e p l a c i n g t h y m u s -d e r i v e d (T) cells in the t h y m u s -d e p e n d e n t a n t i b o d y response to ( T , G ) -A --L of b o n e m a r r o w -d e r i v e d (B) cells in i r r a d i a t e d r e c i p i e n t s (1). T h i s factor was n o t r e m o v e d by a n t i -i m m u n o g l o b u l i n sera ( a n t i m o u s e F a b ) , b u t wasc o m p l e t e l y a d s o r b e d by a l l o a n t i s e r a raised a g a i n s t the H -2 t y p e of the s t r a i n p r o d u c i n g the factor (2). We now report f u r t h e r c h a r a c t e r i z a t i o n a n d m a p p i n g of this T -c e l l factor.

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