Mesenchymal cell aggregates, termed blastema in vivo, precede cartilage differentiation in vivo and in high-density cell cultures. The galactose specific lectin, peanut agglutinin (PNA), has been shown to be blastema specific (B. Zimmermann and M. Thies, 1984, Histochemistry 81, 353-361). PNA appears to be a marker for precartilage cellular aggregates both in vivo and in vitro. Frozen sections of stage 24 chick wing buds were double stained with PNA-rhodamine and by indirect immunofluorescence with antibody directed against type II collagen. The PNA stained the humeral blastema intensely and extended distal to the level of type II collagen. High-density cultures of stage 24 chick wing buds were also evaluated for the distribution of PNA binding. Sixteen-hour cultures showed the earliest consistent appearance of PNA binding. The PNA-stained areas coincided with hematoxylin-stained cell aggregates. PNA staining was inhibited by 50 mM D(+)-galactose and was not sensitive to 1% testicular hyaluronidase pretreatment. No Alcian blue-staining nodules were present yet at 16 hr. The presence of a precartilage, blastema-specific marker in situ, as well as in precartilage aggregates in cultures, suggests the similarities in chondrogenesis between these two conditions. Stage 19 limb bud cultures did not form nodules but did form aggregates that were PNA positive. Furthermore, single cells that differentiated into chondrocytes on collagen gels or after cytochalasin D treatment lacked PNA-binding material. These results suggest that this material is specific to precartilage aggregates. The PNA-positive material was extracellular in distribution and was removed after brief extraction with 0.5 M guanidine hydrochloride.