We present findings regarding the usefulness of urine latex particle agglutination (LPA) testing for group B streptococcus (GBS) antigen, as well as hematologic values, in diagnosing sepsis during the 1st week of life. Of 475 inborn neonates evaluated, 27 (5.7%) had confirmed sepsis, 47 (9.9%) had indeterminate findings precluding further classification, and 401 (84.4%) were not septic. We examined the following: (1) total white blood cell count (WBC); (2) total polymorphonuclear leukocytes (PMNs); (3) total immature PMNs; (4) immature to total (I:T) neutrophil ratio; (5) immature to mature (I:M) neutrophil ratio; (6) platelet count; (7) a 6-point hematologic score based on the preceding values; and (8) urine LPA for GBS. The positive predictive values of the six hematologic values, as well as the hematologic score, were disappointingly low. Using higher I:T ratios (greater than or equal to 0.30 or 0.40) marginally increased the positive predictive values. More than 60% of our ill, nonseptic neonates had I:T ratios greater than or equal to 0.20. Of 15 infants with culture-proven systemic (blood or cerebrospinal fluid) GBS infection who were tested with urine LPA for GBS antigen, only 4 had positive results. Nineteen infants had positive urine LPAs with concomitant negative blood cultures. Urine cultures were obtained from 18 of these infants. No organisms were isolated from any of these specimens. The mothers of 7 of the infants with positive LPAs and negative blood cultures had received intrapartum antibiotics. Of the remaining 12 infants, the urine LPA was repeated in 8 and results were negative in each case.(ABSTRACT TRUNCATED AT 250 WORDS)