Background: Rituximab (anti-CD20 Ab) is the cornerstone of the treatment of non-Hodgkin B lymphomas. Infusion-related reactions (IRR) are the most common adverse effects. To reduce them, intravenous premedication with antihistamine and acetaminophen is administered prior to rituximab. If no IRR after first infusion, subsequent infusions time takes 3-6 hours. Many centers use the rapid 90-minute infusion (off-label). Since 2017 subcutaneous rituximab formulation is available, that takes 5 minutes of administration. Nevertheless, in order to reduce cost, due to approval of biosimilars, some health providers continue using intravenous rituximab. On the other hand, with COVID pandemic, an effort to reduce visits and day-care hospitals stays has been made. In the current situation, it would be convenient to reduce day-care stay and the nursing care burden. We wanted to evaluate the safety of an ultrarapid infusion of biosimilar rituximab in a total time of 30 minutes by analyzing IRR and adverse events (AE). Methods: Since November 2021, 3 cohorts of ultrafast infusion have been studied as follows: One cohort (Cohort 1) with intravenous premedication with dexchlorpheniramine and acetaminophen, followed by rituximab infusion over 1 hour, and 2 cohorts with rituximab infusion over 30 minutes: Cohort 2: with intravenous premedication, and cohort 3 with oral premedication. IRR and adverse events have been independently reviewed and graded using Common Terminology Criteria for Adverse Events (CTCAE) Version 5.0 (November 27, 2017). Results: 34 patients have been included receiving 48 rituximab infusions (16 infusions in each cohort). Median age was 64 years old (range: 51-91). Diagnostic of NHL were as follows: large b cell: 10; follicular: 13; marginal: 7; mantle cell: 1, Waldeström: 1; Ritcher transformation: 2. Rituximab infusion was in monotherapy (21), and in combination (27) with: bendamustine: 9, CHOP: 17, GEMOX: 1. Considering safety, no IRR has been observed in cohort 1 (1 hour infusion), and 1 IRR grade II in cohort 3 (30 minutes, oral premedication). Other AE were: hypertension grade I and hypotension grade I, both in cohort 2. Conclusions: Ultrarapid rituximab infusion is safe. Oral premedication is feasibly allowing a total infusion time of 30 minutes. This infusion rate alleviates day-care burden saving between 75-90% of time in each rituximab infusion, reduce day-care stay and is comfortable for the patients. Clinical trial information: NCT05191225.