The present invention relates to a Lactuca sativa seed designated 41-123 RZ, which exhibits resistance against downy mildew (Bremia lactucae) races B1:1 to B1:28 and CA-I, CA-IIA, CA-IIB, CA-III, CA-IV, CA-V, CA-VI, CA-VII, CA-VIII (Bremia lactucae Regel), and strain FR10.021, currant-lettuce aphid (Nasonovia ribisnigri) and lettuce mosaic virus (LMV), as well as solid main veins and narrow base leaves. The present invention also relates to a Lactuca sativa plant produced by growing the 41-123 RZ seed. The invention further relates to methods for producing the lettuce cultivar, represented by lettuce variety 41-123 RZ.