We report on the construction of promoter probe vector pKWIII, useful in cloning and analyzing strong promoters for Escherichia coli RNA polymerase. Also T4 early promoters that proved to be difficult to clone with other vectors could be tested. The promoter activities obtained with this convenient and nonradioactive system largely correspond to those determined by pulse-labeling of transcripts in the same system. Results with well-characterized control promoters are in good agreement with values given by other authors. We present relative activities of several early promoters of phage T4 and compare these to promoter activities of other phages. Sorting the T4 promoters according to strength suggests the importance of distinct sequence elements to promoter functioning. They are centered around positions -52, -42, and -15.