The IgG Fc receptor II on human monocytes is polymorphic in its ability to bind mIgG1, and its isoelectric focusing pattern. To study the molecular basis of this polymorphism, a cDNA library from cell line K562, expressing two different allelic forms (high responder [HR] and low responder [LR]) of Fc gamma RII, was used for cDNA cloning. We report the isolation and identification of different Fc gamma RII cDNA clones, comprising the LR form of Fc gamma RII, as was evident from studies using a new HR-specific anti-Fc gamma RII mAb 41H16, and from rosetting experiments. Sequence analysis revealed that HR and LR forms differ by two amino acids, both located in the external domain. In the cloned LR form, a glutamine is substituted by a tryptophan residue at aa position 27, located in the first Ig-like domain, and an arginine residue by a histidine residue at aa position 131 in the second Ig-like domain. Furthermore, an Fc gamma RII cDNA clone was isolated with a deletion of 123 bp, overlapping the predicted transmembrane segment. Data showing the presence of an alternatively spliced mRNA detected by using polymerase chain reaction (PCR) might suggest the existence of a soluble form of the human Fc gamma RII, in addition to the membrane-bound forms.