getting a new factor

tmp <- data.frame(G=factor(letters[c(1,2,3,1,2,3)]), 

+                   S=factor(LETTERS[c(1,1,1,2,2,2)]))

tmp

G S
1 a A
2 b A
3 c A
4 a B
5 b B
6 c B

tmp$G.S <- with(tmp, interaction(G, S))
tmp

G S G.S
1 a A a.A
2 b A b.A
3 c A c.A
4 a B a.B
5 b B b.B
6 c B c.B