The theta globin gene is the most recently discovered member of the alpha globin gene family. Its pattern of expression during development is not fully defined, and its encoded protein has not yet been detected in vivo. The detection of theta globin messenger RNA (mRNA) in embryonic and fetal erythroid tissue but not in adults has suggested that theta is an embryonic globin gene. The present study further defines the pattern of theta globin gene expression. We use a modification of the highly sensitive polymerase chain reaction (PCR) technique to assess the levels of theta globin gene expression during development. We confirm the presence of the theta globin mRNA in embryonic and fetal erythroid tissue, and, in addition, we find theta mRNA in the peripheral reticulocytes of normal adults. Furthermore, using the same analytic approach, we detect low but significant levels of the embryonic zeta and epsilon mRNAs in reticulocytes of normal adults. Both zeta and theta gene expression appears erythroid specific in that neither mRNA species is detected in RNA isolated from brain tissue, peripheral blood mononuclear cells, or three nonerythroid cell lines (B-lymphocyte, T-lymphocyte, and hepatoma cell lines). The relative levels of zeta and theta gene expression were assayed during development by a coamplification technique. The results demonstrate the expected developmental regulation of zeta globin mRNA. In contrast, the level of theta globin mRNA fails to demonstrate the significant changes of the magnitude seen in other globin genes and remains low in embryonic, fetal, and adult life. The lack of zeta and epsilon globin proteins in normal adults using highly sensitive immunologic techniques, as reported by others, stands in contrast to these mRNA results and suggests a gap between mRNA and protein expression.