During a 5-month period, 248 general practitioners from 164 general practice offices obtained duplicate throat swabs from 2469 patients with acute pharyngotonsillitis. At the general practice office, one of the swabs was assayed for the presence of group A streptococcal antigen, using one of five commercially available diagnostic kits. (The participants did not receive instruction in the use of the kits). The other swab was cultured in our laboratory for the presence of group A streptococci. The diagnostic parameters for the five kits were as follows: Abbott TestPack Strep A--sensitivity, 79% (110/139) and specificity, 98% (490/501); Direct Strep A EIA (Roche)--sensitivity, 79% (104/132) and specificity, 63% (178/281); Respirastick--sensitivity, 55% (46/84) and specificity, 96% (298/309); Reveal Colour Strep A--sensitivity, 82% (114/139) and specificity, 83% (223/268); Tandem ICON Strep A--sensitivity, 78% (111/143) and specificity, 98% (362/368). The antigen detection test results were heavily influenced by the clinical prediction of a streptococcal or viral etiology. It is concluded that in the absence of a standard throat culture, the Abbott TestPack Strep A kit and the Tandem ICON Strep A kit both offer a more safe diagnostic alternative than clinical assessment. Depending on the current prevalence of group A streptococci, the additional use of a throat swab culture should be considered in case of negative antigen detection test results.