Root nodule specific proteins of Alnus hirsuta were examined. SDS-PAGE pattern of the Alnus root nodule was simpler than that of soybean, showing five nodule specific proteins whose molecular weights were 48, 40, 36, 26 and 19 kD, respectively. Among them, 48 kD protein existed most abundantly and were composed of two subunits whose pI value were 4.0 and 4.3, respectively. The 48 kD protein seemed to be a heme containing protein based on reaction with diaminobenzidine. Although 19 kD protein was present in small amount, it was most similar to leghemoglobin in terms of its molecular weight.