Two dominant mutations at the RseD locus regulating the differential expression of alternative cell surface immobilization antigens of the ciliate Tetrahymena thermophila are described. RseD1 and RseD2 express I to the exclusion of H (28 degrees C) and are leaky for I when expressing either L (15 degrees C) or T (40 degrees C). Complementation was not observed in RseD1/RseD2 heterozygotes, and in 326 testcross progeny no wild-type (micronuclear) recombinants were observed. Macronuclear recombination also was not observed. RseD is located on chromosome 5, at least 50 map units from rseB, which also regulates antigen expression. This brings to four the number of loci known to regulate antigen expression.