A cultured mouse myeloma cell line, S107, that secretes an IgA phosphocholine-binding immunoglobulin has been cloned in soft agar and overlaid with phosphocholine-hemocyanin. Spontaneous mutants that secrete immunoglobulin with a decreased ability to precipitate antigen were detected with this plate assay and occur at a very high frequency. From one such mutant, phenotypic revertants arise spontaneously with a frequency of 0.28-2.8%. This mutant and one of its revertants were studied, and they were found to differ from the parent and from each other serologically and in antigen binding. While it is not yet clear whether these findings bear any relationship to the normal generation of antibody diversity, they do indicate that it is possible to generate antigen binding diversity in somatic cells.