Nonsense suppression by supE44 has been examined in a collection of 14 T4 gene 22 and gene 23 UAG mutants, for which the precise gene location is known. In concordance with previous studies, UAG followed by a pyrimidine was inefficiently suppressed. However, among positions with similar 3' nucleotides, there was considerable variation in suppression efficiency. The competition between supE44 and Release Factor 1 (RF 1) was also investigated following the introduction of a multicopy RF 1 plasmid. An inverse relationship between the efficiency of suppression and RF 1 competition was observed.