The result of a rapid co-agglutination test was compared with the clinical diagnosis of group A streptococcal tonsillitis, a laboratory culture from the throat serving as the correct result. The study was carried out in a family practice, with two GPs partaking: it included 105 consecutive patients. The outcome of the rapid test was significantly correlated to the degree of growth determined by throat culture, and it was superior to the clinical judgement. The statistical analysis of the results of the rapid test revealed (95% confidence limits in brackets) a specificity of 97% (91-100), sensitivity of 68% (48-84), a positive predictive value of 90% (70-99), and a negative predictive value of 89% (81-95). Although the co-agglutination test was superior to the clinical diagnosis, the sensitivity of this test is rather low when compared with other recently evaluated rapid tests for use in family practice.