T h e i m p o r t a n c e of a ce r t a in t y p e of reac t ions , whe re two spacel ike p h o t o n s a re e x c h a n g e d in e lec t ron-e lec t ron or e l ec t ron-pos i t ron collisions, has been s t ressed severa l t imes in t h e pa s t (l-e). I t is t he pu rpose of th i s p a p e r to show t h a t u n d e r ce r t a in cond i t ions p h o t o n p h o t o n collisions can be p roduced , a t h i g h ene rgy a n d w i t h r e a s o n a b l y h i g h c o u n t i n g ra tes , b y us ing t h e new (high-energy, h igh luminos i ty ) e l ec t ron-pos i t ron s to rage r ings w h i c h are n o w u n d e r cons t ruc t ion . One will t h u s be able to s t u d y t he ma te r i a l i z a t i on of 2 p h o t o n s in to va r ious par t ic les , w i t h o u t need ing any special device ( such as sugg ( s t ed b y CSONKA (7)) for t h a t purpose . W h a t we sugges t is t h a t , in a n e lec t ron-pos i t ron collision, b o t h t h e ou tgo ing elect r o n a n d t h e ou tgo ing pos i t ron shou ld be de t ec t ed a t ve ry smal l angles (a few miUir ad ians ) w i t h respec t to t he i r i n c i d e n t d i rec t ions , in coinc idence w i t h o t h e r par t ic les (for i n s t ance , a pa i r of cha rged p a r t i c l e s ) e m i t t e d a t large angle w i t h respec t to t h e co l l id ing-beam axis. The m a i n c o n t r i b u t i o n will t h e n come f rom d i ag ram I ) o f Fig. 1, where two v i r t u a l spacel ike p h o t o n s are exchanged . T h e squa red f o u r m o m e n t a of b o t h these p h o t o n s (q2, q,2) h a v e va lues v a r y i n g f rom a few (keV) 2 to a few (MeV) 2, a n d in fac t (because of t h e q-a, q,-4 fac tors in t h e p ropaga to r s ) t he smal les t va lues will cont r i b u t e t h e mos t . Therefore , we m a y consider t he se p h o t o n s as (~ a l m o s t rea l ~ (in par t icular , we m a y neglec t t he i r l o n g i t u d i n a l com ponen t s ) a n d we m a y t r ea t , u n d e r t h e cond i t ions defined, a n y r eac t ion of t he t y p e e -e + -> e c + A A + as e q u i v a l e n t to yy--> A A + (here A • is a n y cha r ged par t ic le) . W e shal l now show t h a t :