A cDNA coding for a new subunit of soluble guanylyl cyclase with a calculated molecular mass of 81.7 kDa was cloned and sequenced. On the basis of sequence homology, the new subunit appears to be an isoform of the alpha 1-subunit and was designated alpha 2 as the new subunit is very similar to the alpha 1-subunit in the middle and C-terminal part; it is quite diverse in the N-terminal part. Preceding experiments had shown that coexpression of the alpha 1- and beta 1-subunits is necessary to obtain a catalytically active guanylyl cyclase in COS cells [(1990) FEBS Lett. 272, 221-223]. The finding that the alpha 2-subunit was able to replace the alpha 1- but not the beta 1-subunit in expression experiments demonstrates the interchangeability of the alpha-subunit isoforms of soluble guanylyl cyclase.