Runt-related transcription factor-2 (RUNX2) is expressed as two isoforms (type-I and type-II) differing only in their amino terminal sequences. The ami-no terminus of type-I contains MRIPV instead of MASNSLF-SAVTPCQQSFFW in type-II. Although type-II mRNA has been considered osteoblast specific, the RUNX2 protein isoforms expressed in osteoblasts have not yet been identified. Using antisera generated against the two different amino terminal sequences of type-I and type-Il RUNX2, we show the expression of both isoforms in cells with the mature osteoblast phenotype (fetal rat calvarial cells, and ROS 17/2.8, SaOS-2 and U2OS osteosarcoma cell lines), but only type-I in partially differentiated osteoblast-like cells (the LTAM-106 osteosarcoma cell line). Since LTMR-106 cells express both type-I and type-II mRNAs, our results suggest that the translation of type-II mRNA is repressed in these cells. No RUNX1 and RUNX3 proteins are detected in any of the osteoblastic cells tested. The antisera we have generated will be useful for studies relating expression of RUNX2 isoforms to control of osteoblast differentiation.