select options

i have put some values in the select option field material type s_mtart YTRA,YFGC and some other values and i want my selction to be restricted to only this 5 material types

and i am using this code

SELECT matnr mtart FROM mara

INTO TABLE itab_mara

FOR ALL ENTRIES IN itab_receptions

WHERE matnr = itab_receptions-matnr

AND mtart IN s_mtart.

but it is reading all the records

can u tell me why