Previously, the cDNA coding for human low-affinity receptor for IgE (FcERII/CD23) was cloned by three independent groups. Using oligonucleotide probes corresponding to the published cDNA sequence of CD23, we have isolated cDNA clones encoding FcERII from the cDNA library of B-lymphoblastoid cell line 1B obtained in our laboratory. Cloned nucleotide sequences of FcERII were shown to be identical to those from the RPMI8866 cell line. Using a DNA fragment containing the full-length cDNA copy of CD23 as a probe, CD23 expression in highly purified peripheral blood B lymphocytes was shown to be induced by recombinant human IL4 in a clearly dose-dependent manner.